Front projectors really are only good in a room with very little to no ambient light, so I would eliminate that possibility right off the bat.
For a flat screen, you could make a hinged drop-down mount for the ceiling. That would stow it up out of the way. If you got really fancy you could make a box enclosure around the back of it (assuming the TV folds up face first against the ceiling). That way it would help protect it when stowed away, plus make it less obvious that it's a TV, except for the cables running up the wall and ceiling to it. However, if it's a relatively low ceiling, that may not work at all.
Another option might be to make some sort of dartboard type cabinet for it that can be locked up. Just mount the TV to the wall using a regular wall mount, and then build a nice, strong box around it with strong locking doors.
That's the best I could come up with.