You could do this by creating your screensaver in Flash, or Director, with the video embedded, then create a projector from the flash/director file.
Then use a software Like flash jestor, or cinemac to convert the projector file into a screensaver. - works in Os 9, not sure about how OS x screensavers work.
You could also have the sound in the screensaver too, with this method!
let me know if this was any use.
Steve