As a reference point, the previous owner of my house had hung the garage door opener by a couple of "toggle bolts" into the dry wall and that held up. However, I used angle iron screwed right into the joists with lag bolts when I replaced it.
Similarly, if it was my new and expensive toy, I would overengineer the mount. You could try screwing a large flat wooden plate into the joists and then screwing the projector mount into that. A 24"*24" (or so) square sheet of 3/4" plywood would probably do the trick. Paint ot taste. Four 3" #10 screws should provide plenty of holding power. Put one pair into each ceiling joist. In typical house construction the joists should run parallel and be spaced on 16" centres. Visualise this when finding them.
If you were super worried about appearance, you could figure out where the joists were, then cut the plywood oversized so that the projector would be centred on the plywood. Your average Home Depot should be able do the cuttting for you.
Happy mounting !
Brent