Atsuji Tetsuji's Bushu no hanashi 部首の話 ("Talks on Radicals") mentions, as a interesting tangential remark, that the pronunciation of 卍 in Japanese is manji because it means "man (万/萬, 'myriad') character (字, ji)". I'd never actually thought about this before; I suppose this means that manji isn't actually its pronunciation, but its name.

Poking around for more information, I found this passage in the Fanyi mingyi ji xu 翻譯名義集 ("Collection of names and meanings in [Buddhist] translations"), itself quoting a document called 華嚴音義, presumably some sort of commentary on the Avataṃsaka Sūtra (I haven't been able to identify it precisely):

T2131_.54.1147a04: [...] 案卍字。本非是字。
T2131_.54.1147a05: 大周長壽二年。主上權制此文。著於天樞音
T2131_.54.1147a06: 之爲萬。謂吉祥萬徳之所集也。

Which I think means, roughly: "The character 卍 was not originally a character. In Changshou 2 [693 AD] of her Great Zhou Dynasty, the Empress [Wu Zetian] provisionally declared that this symbol would represent (? 著於 = be represented by?) Dubhe with the sound 萬, (wàn in Chinese, man in Japanese). It means 'the gathering of auspicious signs and the myriad virtues.'"

So does this make it one of Wu Zetian's characters? I should note that I wasn't able to find any evidence that she ever declared it a symbol of the sun — all the sources I found were quite clear that she associated it with Ursa Major. Does anyone know where the sun claim comes from?