Hi Paul,
Currently, the audio and video in BigBlueButton follow two separate paths. We use red5 for relaying and recording the video streams (including the desktop sharing). We use FreeSWITCH for mixing and recording the audio.
For connecting the video stream, you would need to pull out the stream from red5 (so users coming in through H.323 could see the BigBlueButton users) and injecting a stream into red5 (so BigBlueButton users could see users coming in through H.323).
To our knowledge, no one has attempted this integration. From the perspective of the core developers, there have always been higher priorities (such as record and playback, HTML5 client, etc).
Regards,... Fred
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BigBlueButton Developer
BigBlueButton on twitter: @bigbluebutton