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.