I don't remember seeing those used either, but they're definitely in Unicode, and early enough to be supported by most readers. That's also how I use oddball characters, though, like when I had a character named Nyx Pi (spelled out since I'm on my tablet and don't know quite how to insert Unicode glyphs into posts on it). She was an engineered human; the first name was the model identifier and the second was usually their serial number in hex. Prototypes were given irrational number symbols as serial numbers; there was also a Nyx e (as in Log Natural e, which is also irrational). As a result, I had to keep the Greek letter on the clipboard and paste it in whenever I needed it.
Somehow it also made me think, "The umlauts are starting at you!"