Usernames aren't limited to ASCII, as you can also register Chinese usernames (for instance, try ä½ å¥½ ). I suppose it's possible to register usernames containing emoji unicode characters (
). However, these usernames cannot be registered. The form disallows the use of these characters, and maybe there are also server-sided checks. In order to allow these characters, removing the checks (unsafe) or adding emoji characters to the checks (safe) from the source code would suffice.
But keep in mind that the character
is different than
, as the first one is unicode and the second one an image. Whatever emoji you want to use, it will have a unicode equivalent.