Since Octane uses real world lighting, I had to model a projector system that works similarly to how it does in the real world. Basically a light source behind a specular screen with an image in its transmission node. Though if you just put any old light behind the screen, you'll only get a fuzzy image. (Daylight might work, but it overpowers everything else.)
I'll explain how I managed to get a crisp projection image when I upload my projector prop.
(As for elves...probably a lot. lol)