You can not log into Facebook Chat via XMPP with an email address – however I might be able to take said email address and look up the username via FQL. Part of this will also include sending the user back an "omgwtfbbq" if they haven't set up a username in facebook yet.
Another part of this will involve sending a different error back if the username is non-existent. Perhaps <registration-required/>
The more I think about this the more I'm inclined not to support it. It's not like Pidgin and such supports this, and on top of that it's an ugly hack to get this to work so I'm not sure I want to actually do it. Postponing.