Large scale video conferencing
I've messed with BBB, though no where close to those numbers.  As the presenter, you can prohibit participants from sharing their video or audio, making it a one way presentation.  

I have a gut feeling though, that the server requirements would be significant for that many participants.  We ran it on an old HP G3 and it worked , but slow for a dozen participants.

From their FAQ:


  What are the minimum hardware requirements for the BigBlueButton
  Server As a minimum, we recommend you run BigBlueButton on a dedicated
  quad-core machine with at least 4 GB of memory and a 2.6+ Ghz
  processor.
  
  We recommend installing BigBlueButton on Ubuntu 10.04 32-bit/64-bit on
  the server (see Installing BigBlueButton).
  
  While the BigBlueButton VM is a good way to try out the system, we
  recommend you run BigBlueButton on a dedicated server to reduce audio
  lag. In a virtual environment, the BigBlueButton VM gets only a
  portion of the host's CPU, and any lag in the CPU cycles can translate
  to lag in processing VoIP packets.
  
  What are the minimum bandwidth requirements for the BigBlueButton
  Server You'll need good upstream and downstream bandwidth from the
  server. We recommend (at least) 100 MBits/second bandwidth in both
  directions.


Personally, I'd probably looking into a SaaS solution like Webex, GoToMeeting, or the like.  There are probably at least a half dozen solid options.  Without having done it before, I'd hate to have sole responsibility for a conference that big.