6 years after the initial post and 5 years after the SOXMPP project, I've decided to develop one myself.
It's based on SleekXMPP and ChatExchange python libraries and it currently only supports servicing one room and one stack/xmpp user per process.
I've posted more about XMPPExchange on StackApps, and if you'd like to just jump into Github, check the code ...
Working on an XMPP to google-chat bidirectional pipe.
Still a work in progress, but I just programatically drive a firefox browser that's rendered in xvfb with Selenium. Then screen-scrape it. It's here. Not fully functional yet, but it is a work in progress.
The passing to another protocol thing I'm trying to set up to be pretty modular.
In my ...