I did something like that using a lift (drop?) from Auton. It was the type that used what looked like a pair of roller shades that supported a platform in the middle, where the projector sat. The bottom of the platform was covered with T&G to match the ceiling, which looked really nice. The hardest part was getting the projector to drop consistently. The rolled material wanted to walk if the lift unit wasn't perfectly level, which would push the image off the screen. But we eventually got it.

EDIT - IIRC, I think we dropped the projector about 24". It was a pretty good amount, actually.