@gargron Either way, I don't think there would be a problem for !GNUsocial with remote #Unicode (non-ASCII) characters. Nicknames are normalised as-is (for example removal of underscores which was another discussion). I don't remember if transliteration of nicknames is done, but we do that for #håshtägs (which has also sparked controversy due to incompatibility with farsi).
My desire is to implement unicode support for nicknames. I don't know about the best strategy for homoglyph attacks etc, but I bet the !xmpp community has lots to share there. There you can have at least a large set of Unicode in your user part of the XMPP ID. Maybe #Prosody developer @zash knows more about unicode in XMPP usernames etc?
cc: @munin @nightpool