This is an interesting question you asked. I’'m wondering if the jumbled up characters you are seeing is client issue rather than a server issue. The reason I suspect that this might be the case is that Messenger (and I would suspect most other XMPP/Jabber based servers) does not really read or do any sort of transformation to the body of a message packet. Like an email server, Messenger just looks at where a message wants to go and routes it as need be.
I’‘d be curious to know where along message route the characters are being jumbled. I’'m not familiar with Gush but I know with Miranda there is a way to turn on debugging where you can watch the xml traffic that is being sent/received. Also, you can turn on the message auditing feature in the Messenger Admin Console to see what the packets look like going to/from the server. So with all that debugging turned on you could figure out pretty quickly where the messages are getting jumbled.
Give it a shot and report back what you find.
Hope that helps,