I'm having a feeling I'll regret having done this, but xochat.org is now up and running. If you've got an XO you can point it at my Jabber server by popping open a terminal and typing this:
sugar-control-panel -s jabber xochat.org
Then hit ctrl+alt+erase to restart Sugar. You should then see the other people logged in to the server in the "Neighborhood" view. That is essentially the widest view. You should be able to see other users and shared activities, and share your own activities with them, including video.
This is, however, the bleeding edge of the bleeding edge, so don't expect perfection.
Also, this is a cheap shared server account, so I have a feeling it will fall over pretty quickly. Please don't trumpet this on your blog; if I get 1000 users tomorrow I'll probably just have to turn the whole thing off and start over by invitation only, but we'll see. I have no idea, really.
And, while you can connect to the server with a regular Jabber client, it doesn't appear that you can talk to XO users with it. Also, it seems like you should be able to connect with an emulator, but it didn't work for me.