RTMP does not allow multicast natively and it will be a very painful project to implement multicasting by yourself on client side. But there are two solutions :
- You could try RTMFP multicast serverless solution. Here is a good example explained by Tom Krcha : http://tomkrcha.com/?p=1803 (just add a NetStream and attach the camera to it),
- Otherwise you could try Adobe Media Server or MonaServer (our open source server) to use NetGroup and RTMFP. I think that re-implementing the server part of Bigbluebutton should be easy.
EDIT: RTMFP with NetGroup is the only simple solution to have multicast (P2P broadcasting) on the internet.