TellyMate Shield - TV output for Arduino

I love the idea of the TellyMate Shield. I may need to buy one, but I've got one question... What about adding an LM1881 or LMH1980 sync separator to allow the TellyMate to overlay on top of pre-existing video? I'm honestly not sure how hard that would be to do, but it would add a whole new world of useful applications to the TellyMate. For example, live GPS overlay over a video signal.