I tried doing a similar thing a while ago. One of the problems I ran into was rendering plugin regions (flash,java, etc.). The problem was, as I recall that the ns plugin interface passes plugins a Windows native window handle, rather than a DC/region to render to.
If you've made any progress in this area, I would be very interested in discussing it further.
Right... unfortunately I haven't solved that. But for the project I was working on, it was sufficient to just ignore plugins. Since you're really not doing anything interactive in an offscreen render, it seemed sufficient to ignore most plugins as they require interaction (what do you render to a bitmap for a flash presentation?)
