Test: Zalgo in a username
Result: Gets blocked with invalid character rule in the first place, likely impossible.
Just remembered that in a past test, I tried out the Zero-Width Non-Joiner, a character that is completely invisible, as a username.
IIRC, it also got rejected for that.
Think they have limited the acceptable usernames to a small amount of allowed characters.
EDIT: I just tested some german specific characters only (äöüß) and also got the rejection, so it's likely only the basic letters and numbers are allowed for usernames