Reading the question "Have there been efforts to introduce non Greek or Latin alphabets into mathematics?" at Mathematics Stack Exchange, I noticed this in an answer by Dan Peterson:

Let X be a quotient of a bounded symmetric domain by an arithmetic group. [...] Namikawa tried to popularize the notation X^サ for the Satake compactification. サ is katakana, the first initial of Satake. It did not stick.

Intriguing! I tried to find more on this, but the closest I got was Yuji Odaka's "Tropically compactify moduli via Gromov-Hausdorff collapse", which uses the notation X^さ, with a footnote:

The character "さ" is Hiragana type character which we pronouce "SA", the first syllable of Satake and the idea of using this character is after Namikawa’s book [Nam2] which used Katakana "サ" instead (but we japaneses rarely use katakana for writing japanese name). The corresponding Kanji character 佐 is more normal.

So apparently part of the reason it did not stick is disagreement even within the Japanese-speaking mathematics community over which type of character to use. (You see? East Asian orthography really does retard the progress of the sciences!)

"[Nam2]" is a reference to this book:

[Nam2] Y. Namikawa, Toroidal Compactification of Siegel Spaces, Lecture Notes in Mathematics, vol. 812 (1980).

... which is, alas, not in any library collection I have easy access to and too expensive to buy for blog research. Here the trail went cold, in other words.