I have been using OpenTok for developing a webapp that can broadcast a stream to upto 2500 subscribers. OpenTok handles most of the work for you, but it doesn't allow screensharing - which is critical for my webapp. To enable screensharing, I'm looking for open source plugins/drivers which I can ask broadcasters to download and install, and the plugin should capture the screen and share it as a virtual webcam so that services like OpenTok can broadcast/archive/store it.
How do you proceed to write such a driver? Do you know any open source project which can serve as a good starting point?