Nihongo visual kei 日本語ビジュアル系, by "subculture linguist" AKIZUKI Kōtarō 秋月高太郎, examines relatively recent trends in casual Japanese orthography against the larger history of the written language. So, his discussion of small-tsu as an emphasis/intensity marker (think "あっ" vs "あ") also goes over the history of gemination in Japanese and the various strategies devised to get it on paper.

Akazuki formalizes the rules for what he observes carefully, but there isn't much in the way of statistical analysis or close-range orthographohistoriography: we learn that teenage girls like to use the small version of characters like あ and や, but we don't learn how long this has been going on, who started the trend, if it has a geographical component, or anything else, really, except for some hints that maybe it was the gyaru. (It usually is.)

The book also include at least one completely baffling passage in the introduction. After listing a few examples of mangled Japanese on signs aimed at Japanese tourists outside Japan (mostly drawn from Nihongo de dozuzo), Akizuki writes:

If Japanese people were to write Arabic in Arabic script, they would quite possibly make the same sorts of mistake. We cannot point the finger only at foreigners. However, one thing does strike me: the casual use in public, eye-catching contexts like signs and notices of text that could be mistaken. It seems to me that to Japanese people, or in Japanese culture, to make an error in written text is an extremely embarrassing thing to do. Most Japanese people, if they were to learn that the writing on their sign was wrong, would fix it right away, or take the sign down... (おそらく、日本人がアラビア語をアラビア文字で書いたら、同じような間違いを犯してしまうことはありえるでしょう。外国の方を一方的に責めることはできません。ただ、ちょっと気になるところがあります。それは、間違っているかもしれない文字表記を、看板や注意書きのような、目立つところに、平気で出してしまうという気持ちのもちようです。日本人、あるいは日本文化において、文字を間違えるということは、かなりレベルの高い恥に属する行為ではないでしょうか。日本人は、もし、看板の文字が間違っていることに気づいたら、すぐに訂正したり、その看板をはずしたりする人がほとんどではないでしょうか。）

I understand that his point is that Japanese people care about writing and so it is of interest that so many of them are using this "new orthography." But I live in a Japan where the signage reads "Let's we are enjoying good law-men times" and the t-shirts say "The beautiful girls defintion [sic!] is changed," and no-one seems in the least embarrassed. Not that I think they should be — words as decoration, it's cool, I'm hip — but it's not easy to square this with Akizuki's Gedankenexperiment.

A couple of other interesting observations from the book:

• On pp253-255, Akizuki observes that the huge gulfs of white space employed in keitai shōsetsu are apparently rendering the period 。 irrelevant in that context, although exclamation and question marks remain at full strength.
• On p116, he quotes KINDAICHI Kyōsuke 金田一京助 on why the particles は, を and へ retained their non-phonemic spelling. "Ideally," Kindaichi wrote, "Everyone would like to change を to お," i.e. write the particle phonemically. But particles are used so frequently that to modernize them would be "too great and distracting a change" from what people were used to, and the idea was abandoned.

So The Battles of Coxinga is a Chikamatsu jōruri play loosely based on the life of Zheng Chenggong 鄭成功 a.k.a. Guóxìngyé 國姓爺 or "Koxinga." It is 100% badass, opening in the Ming Empire with a scene in which the Minister of the Right gouges his own eye out to prove his loyalty — and following that up with the big reveal that the eye-gouging was actually a secret message to the enemy. (The events of the play are set before the invention of hand signals.)

Anyway, to cut a long story short, Tartars invade the imperial residence. Coxinga, newborn son of the betrayed emperor, is spirited away to Japan amid much grue by a loyal retainer who then settles down with a Japanese wife and poses as Coxinga's father. Coxinga grows up, marries a wife of his own named Komutsu, and lives life as a simple Japanese fisherman — until the day Princess Sendan, another refugee from the Ming court, turns up on shore in a Chinese boat. There is some expository husband-and-wife talk ("You think she's pretty? I guess you'd prefer a Chinese wife?" — seriously) as she approaches, and then she finally speaks:

「日本人／＼。なむきやらちよんのふとらやあ／＼」とありければ。小むつぷつと笑ひ出し。「ありやなんといふお経ぢや」と腹を抱へてをかしがる。「ヤイ／＼笑ふな　あれは日本人ここへおぢや。頼みたいといふ事」と押しのけて立ちよれば。上臈涙にくれながら。「たいみんちんしんにようろ。君けんくるめいたかりんかんきう。さいもうすがすんへいする共こんたかりんとんな。ありしてけんさんはいろ。とらやあ／＼」とばかりにて　またさめ／゛＼と泣き給うへば。小むつは浜辺にころりと臥し　腹筋よつてたへかぬる。

"Japanese, Japanese, namu kyara chonnō tora yaa yaa!"

Komutsu burst out laughing, nearly doubling over. "What kind of sutra's that supposed to be?" she said.

"Hey, stop laughing!" said Watōnai. "She said 'Japanese, come here, I have something to ask of you.'" He pushed his wife aside and approached the woman.

Blinded by tears, she spoke. "Ming Empire ching shin nyō ro. Prince, keng kuru mei taka ring kang kyu. Sai mō suga sun hei suru with kong taka ring tong na. Was ken sang hai ro. Tora yaa yaa!" With this, she began to sob again.

Komatsu rolled about on the beach, clutching her stomach and laughing so hard she almost couldn't bear it. But Watōnai, who recognized this as the Chinese his father always spoke, dropped to his knees and lowered his head. "Usu usu usasu wa mō. Saki ga ching buri kakusaku kin nai ro. Kin nyō, kin nyō..."

First observation: Komutsu is a total asshole.

Second observation: Sendan is speaking Chinese-sounding gibberish so meaningless that it's difficult to decide even how to Romanize it. SHINODA Jun'ichi 信多純一's notes for the 1986 Shinchō edition (which is also where I got the Japanese text above) observe that Sendan's opening line is probably a reference to the Nīlakantha dhāranī, which in Japanese opens with 南無喝囉怛那哆羅夜耶 Namu karatannō tarayaya, a transliteration of Namo ratna-trayāya ("Adoration to the Three Gems") if Wikipedia is to be believed.

Note that there are some understandable words thrown in there too: たいみん is "Great Ming", i.e. the Ming Empire (currently, you will recall, overrun by Tartars), and 君 is as in ruler or prince.

So a localization (as opposed to translation) might start something like:

Anglicus! Anglicus! Ava mara gratius planginus domino tecmo! ...

I looked up how Donald Keene translates this part, and found this:

SENDAN: Japanese! Japanese! Na mu kya ra chon nō to ra ya a ya!
NARRATOR: Komutsu bursts out laughing.
KOMUTSU: What sutra is that?
NARRATOR: She holds her sides with amusement.
WATŌNAI: You mustn't laugh! She said, "Japanese, come here. I have something to request of you."
NARRATOR: He brushes Komutsu aside and goes to the lady, who is blinded with tears.
SENDAN: Great Ming chin shin nyō ro. Sir, ken ku ru mei ta ka rin kan kyū, sai mō su ga sun hei su ru, on the other hand, kon ta ka rin ton na, a ri shi te ken san hai ro. To ra ya a ya, to ra ya a ya.
NARRATOR: These are her only words before she melts into tears again. Komutsu plops down on the beach, convulsed with laughter, unable to endure more. Watōnai, who learned his father's tongue, touches his hands to the ground and bows his head.
WATŌNAI: U su u su u sa su ha mō, sa ki ga chin bu ri ka ku san kin nai ro. Kin nyō, kin nyō.

In 共 Keene sees "on the other hand" where I saw "with". On the other hand, he does not see ありし of worthy of translation at all, while I read it as "was". Shinoda lists a couple of potential hidden meanings that neither of us bothered with — speaking only for myself, the reason for this was that I didn't even notice them. For example, I saw Watōnai's "usu-usu" as a partial nonsense gag, but the possibility of "u-wa-sa-chi-ri-sa-n-ro" ("I have heard about you") being intentionally embedded in what comes after never occurred to me.

Reading Don SANDERSON's translation of KATŌ Shūichi's History of Japanese literature, I ran across this in volume two about the 18th-Century beef between MOTOORI Norinaga and UEDA Akinari:

On his portrait Norinaga wrote the following well-known poem:

If someone asks
What is the heart of Yamato:
Shining in the morning sun,
Blossom of the mountain cherry.

Akinari recognized that other countries had their own myths and he thought it impossible to transfer Japanese myths to them: 'Each one of the writings has a separate account of the creation of the universe for each country ... and even if one transfers them to other countries they would not be accepted, being self-regarding accounts.' This is all the more so when Japan, as seen on Dutch maps of the world, is no more than 'a small island, like a tiny leaf that has fallen onto a vast lake'. It would be difficult to persuade other countries that this was the country from which came the sun and moon, with whose light they were all blessed. Akinari notes in his Tandai shōshinroku that it was only 'old' takes that give the sun and moon human forms; in fact 'seen through the telescope they call a zongarasu the sun, which flames, and the moon, which boils, are nothing of the sort'. He dismisses Norinaga's theories as 'the talk of a sheltered rustic' and 'the cant of an indigent priest'. 'The "Japanese spirit" is something without meaning. In any country the "spirit" of that country is its stench.' He adds a verse:

Again all that mumbo-jumbo
About the heart of Yamato
And cherry-blossom.

Norinaga's verse was popular in the militarist Japan of the nineteen-thirties and forties and remains well-known today, while few people are familiar with Akinari's poem. [...]

I decided to look up the original of Akinari's commentary, and, courtesy of the Norinaga Kinenkan and the Kanazawa College of Art (who have both put bits of it online, with commentary) here it is:

月も日も、目・鼻・口もあつて、人体にときなしたるは古伝也。ゾンガラスと云ふ千里鏡で見たれば、日は炎々タリ、月は沸々タリ、そんな物ではござらしやらぬ。い中人のふところおやじの説も、又田舎者の聞いては信ずべし。京の者が聞けば、王様の不面目也。やまとだましいと云ふことをとかくにいふよ。どこの国でも其国のたましいが国の臭気也。おのれが像の上に書きしとぞ。
    敷島やまと心の道とへば朝日にてらすやまざくら花
とはいかに／＼。おのれが像の上には、尊大のおや玉也。そこで、「しき島のやまと心のなんのかのうろんな事を又さくら花」とこたへた。「いまからか」と云うて笑ひし也。

Akinari seems particularly amused by the fact that Norinaga wrote the poem on a portrait of himself. He also misquotes the poem slightly as "If you ask the way of the heart of Yamato, it is the morning sun shining on the blossom of the mountain cherry" rather than "If someone asks what is the heart of Yamato, it is the blossom of the mountain cherry fragrant in the morning sun" (敷島のやまとごゝろを人とはゞ朝日にゝほふ山ざくら花), which is the version you usually hear. (Note that this error seems to have spread in part to Katō's own quotation of the poem as well, with the fragrance-free shining.)

TL;DR Here's Akinari's answer poem, with my own translation:

しき島のやまと心のなんのかのうろんな事を又さくら花
The heart of Yamato, blah, blah, all that crap, and again with the cherry blossoms.