here is nothing...
Do you like to add a hyper link to the XMPP chat room?
Our room is <a href="xmpp:email@example.com?join">firstname.lastname@example.org</a> You are welcome to join and communicate with us.
Copied from: http://gajim.org/dev.php?lang=en
I have found these pages (mailto: irc:// news:// are possible but I do not see XMPP)