Come to think of it,
There must be someone out there in the big bad world of internet-dom who has done this already. I've done a quick search on google and haven't come up with what you're after yet, however there were lots of hits, so I reckon if you fiddle with your search, you should be able to find it. The biggest problem will be finding a video file for which transparency can easily be set. If you don't find it in a couple of days, let me know and I'll sit down and I should be able to do something up in about an hour, because it's something that I'll probably use in the future at some stage anyway. If you do find something, let us know where it is.