This is so late I doubt anyone will see it, but I finally have an answer, thanks to Saul Ibarra Corretge. The missing link is WebRTC - http://www.webrtc.org/ h

This is so late I doubt anyone will see it, but I finally have an answer, thanks to Saul Ibarra Corretge. The missing link is WebRTC - http://www.webrtc.org/ http://www.webrtc.org/ You can set up a connection using SIP (see above) and then WebRTC will provide a Javascript API for making the connection between peers (and the WebRTC docs include various references to ICE which makes me suspect they will work correctly behind NAT etc). So it's possible with experimental Google APIs today ("Latest Google Chrome (dev channel) comes with WebRTC support" - Saul).