Is it safe to hang a 39lb projector screen from a drywall ceiling without using the joists?
Absolutely not. 

First, the drywall will almost certainly sag over time and leave a permanent bulge. That's if (IF) the drywall doesn't pull off the screws holding it to the ceiling. That is the risk of this scenario--not simply the projector coming down. 

Here's how it could play out: You hang the projector, and all seems well. Over time the natural vibration in the home causes the single drywall screw nearest the projector to work through the thin paper top layer of the drywall. Because drywall is flexible, the sheet starts to sag. Now the force is on more screws, but the initial damage is done. If there are only a few nearby screws, the same thing happens there. If you're lucky, this stops movement, but now your projector is hanging on an inverted drywall bubble, bouncing around with every step on the floor above. It's not a pretty situation.

Use a decorative board of some sort to span between joists, and mount the projector to that.