Free one? a free what? Video file?
I'd say just make one using like iMovie or Windows Video Maker (or whatever its called). Or make a bunch of black background images with lots of dots and lines etc., then make a slideshow of those images and burn it to DVD or just play it from your computer.
Smoke & Mirrors, like you said regarding buffo's point - it wont be an issue of "projector audience scanning" However, looking into the projector at a close range would be dangerous. Look at the MPE thing this way, a laser can be completely legal to use without a variance (look at a few crappy DJ lasers - think American DJ, chauvet etc.) even above 5mw. But in those cases there is a lens that always expands the beam. A video projector always has its power spread out... But focus the light out of a projector and you could easily burn stuff with it.