The last couple of days I've been hacking on a small script that lets users use the shoutbox via a jabber/xmpp client.
It's not a real gateway though. It's a simple client that connects to an xmpp conference room as a user. Then it inserts everything written in the room to the shoutbox, and everything written in the shoutbox is sent to the chat room.
If a user has entered their Jabber ID in a profile field and are using that id to chat, the messages will be shown as that user. Otherwise the messages will be entered as the anonymous user, with the JID as the name.
Messages from the shoutbox sent to the chat room will be prepended with the UBB.threads username.
The script requires Python 2.6 and a couple of libraries (MySQLdb and Twisted).
The script can be found here:http://github.com/ollej/shoutbridge
There's still a few kinks to work out, but it works more or less right now.