I think there have been people using a TV out module or a MAX7456 chip to display content over a video stream. Another person here, AndrejK, has talked about it in this thread [url]http://www.tinyclr.com/forum/6/1609/[/url] at least.
Personally, the one I would wait for is the VGA output module GHI have been talking about - there’s a thread on this here somewhere too. It looks like an easy way to do what you want - but I can’t remember if it was intended to be used on the smaller devices or not.