Would it be possible to project onto a ceiling? Perhaps with a mirrior, similar to the mirrir accessory for InFocus' IN1? Or would it be possible -- and I'm sure this is a long shot -- to include something in the build of a DIY projector that allows you to go from wall to ceiling projection?

I did something like this with my 7" but my mirror was after the triplet so if I wanted it on the ceiling I just removed the mirror. It would have worked great but the problem was I was projecting so close it was way out of focus. Meaning I went from like a 10 foot throw to about a 5 foot throw. It was a tall box.

Build a 'straight through' and stand it on end pointing at the ceiling ...... done. Make certain that the vents and fans are not on the bottom if you do this, obvious I know but!!

Get the screen (ceiling) distance right and build accordingly, use the focal calculator.

This is pretty easy, I built a vertical 17" and when I was playing around I removed the mirror and mounted the projection lens on the top and projected onto the ceiling. It would be best (in my opinion) to use the standard short throw lens as you will have such a short distance to the ceiling (unless you have real tall ceilings).
Good luck and let us know how it works out.

I was considering doing that for a while. That way I could have a normal projection (on the wall) during normal use, but I could project onto the ceiling when I wanted to watch TV in bed.

I think the image will be reversed when you go from wall to ceiling so you'll need an LCD controller that'll allow you to flip/mirror the output (like the G2A).