That's precisely how the old ATI TV tuner cards worked. They masked part of your display and any pixels that were the mask color became the video player, because the decoding and injection into your video signal was happening in hardware on the tuner card, not on your regular graphics card.
This allowed you to do dumb stunts like scribble hot magenta areas anywhere on your screen with MS Paint and the scribbled areas would magically become video from the TV tuner.