I wanted to do the same thing, since we'll thinking of selling our house soon, and didn't want to have an "ugly" projector mounted to the ceiling.
What I ended up getting were three Peerless Video Poles, which I had seen on a link in another forum. I used two for my rear speakers, and the third for my projector. To mount the projector, I just drilled some holes on the platform and attached the projector.
Since the pole for the projector has to be mounted upside down, I'm not sure if it will be as stable as the ones I'm using for the rear speakers. I think I'm going to ceiling-mount the projector after all, but in your case this might be a good option.
Let me know if you'd like more info,
Scott