So a couple of weeks ago I had a casual use for hentaigana and went looking to see if there were Unicode code points. Alas, there were none, but through Wikipedia I found that there was a proposal to include it. The proposal document includes relevant uses such as transcribing old documents, restaurant names and documents on calligraphy. Unfortunately the proposal hasn’t been accepted yet, but it did lead to an interesting discovery.
The containing directory has all of the Unicode proposals, some of them labeled urgent because characters have become common use. For example, http://std.dkuug.dk/JTC1/SC2/WG2/docs/n4584A.pdf calls for the addition of several Chinese characters for scientific use, such as 鎶, which is used for the new(ish) element Copernicum, some other weird-looking ones. http://std.dkuug.dk/JTC1/SC2/WG2/docs/n4583.pdf is a request for special Chinese characters that were used by a Russian mission in Beijing to transcribe Slavonic sounds. http://std.dkuug.dk/jtc1/sc2/wg2/docs/n3734.pdf requests an Arabic character used in Urdu for a specific Indian calendar.
Also, apparently two years after the hentaigana proposal was shelved, the Japanese government sent word saying they actually needed it to encode names of some people born before 1948. Now that’s interesting! I’ve never met someone with a hentaigana name.
Each of these comes with cultural background, descriptions of the required glyphs, and pictures or screenshots of the glyphs in the wild.