There's a reasonable cross-section of clients for most platforms the
supports a set of mostly interoperable codecs and transports. It is
possible to source with real/darwin streaming server/videolan a source
that will be visbile to users of quicktime/real/vlc and some other clients
via multicast or unicast transports.
Right. And unless I'm mistaken, streaming servers will happily take either a unicast or a multicast feed and reflect this feed over one of several transports (including some that will bypass NAT).
The transport is an issue. 500Kb/s isma mpeg-4 streams have a real cost if
you want 200 of them...
That's 100 Mbps. There are a lot of outfits that care about the IETF for which 100 Mbps is small change. (The latest episode of the Steve Jobs Show had 60,000 people watching at 300 kbps...) And 200 x 24 kbps audio only is just 5 Mbps, which would even be doable from the meeting site. A reasonable charge to cover the costs for online participation wouldn't be out of the question either, IMO.
The thing I consider most unworkable frankly is low-bitrate video... I
don't consider a 40 80 100Kb/s streams terribly usable regardless of the
codec chosen, I want to be able to read the slides, I want to be able hear
the speakers from someplace other than the bottom a barrel and I want to
be able to discern who's standing at the mic.
Our little multi6 experiment taught me that low quality video indeed isn't all that useful, and goood quality video isn't simple. However, workable quality audio is both simple and useful. So if good video can't be done, forget about it altogether and do audio only. If the speakers get in the habit of putting their slides online before a session and either they or the jabber scribe say which slide is up, that part is covered.
Another option would be "slow scan tv": rather than stream relatively low quality moving video, why not send out periodic high quality stills? The advantage here is that there is no set rate at which those have to load so there is no binary good/none quality problem as with streaming.