Related to bug 33048, I set my alias to 😼 (U+1F63C CAT FACE WITH WRY SMILE) and tried to join a MUC.
M-Link correctly rejects my request to join as follows:
_end_element_ns: Received stanza
* presence xmlns='jabber:client' type='error' firstname.lastname@example.org/Queeg' email@example.com/😼'
* x xmlns='vcard-temp:x:update'
* x xmlns='http://jabber.org/protocol/muc'
* error code='400' type='modify'
* jid-malformed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
But we shouldn't ever get this far!
(This does raise the question of how we represent this restriction on romo nicknames. We arguably need this for IRC too.)