I think the OP wants to simulate the situation like: as if the stuff he sees on his desktop (screen) is actually coming from a (web)camera. I.e. the video card is the source (the camera) and this mysterious 'device' is something like an editing machine or whatever. Kinda like using `recordmydesktop` and piping the output to some external device?
Maybe not though, and some further clarification (including the goal of this project) might help, if this is not the right idea.
Sasha