I keep thinking that I remember hearing that meebo was actually using this plugin at one point or maybe not. Maybe not though.
Either way, the only thing required to use this plugin to interact with those other networks is that you must have an account on the actual networks. (i believe the same goes for meebo) In other words you can’'t arbitrarily send messages to the other networks, you must actually log in with a real account,
Long story short, I -think- this plugin is what you are looking for. =)
BTW, if you are looking for something at a lower level than having an XMPP server running, I would recommend the jclaim library. (go to www.sf.net and search for jclaim) It’‘s a library/framework written in java that should interact with all of he services you mention and it’'s very actively maintained.