My situation is similar-- I don't have a dedicated game room yet, so I either have to be able to set up and take down my rig on game night, or start looking for a new wife. (Hmm...)
So here's my solution:
My projection surface is a lightly-frosted "white" glass tabletop from IKEA. It is about 30" x 60" and stands about 36" off the floor.
If it isn't
this exact model it is one very similar.
I bought a wide (24") mirror and mounted it to an apparatus composed of two 24"x24" particle boards hinged together along one edge. It sits on the floor like an "L". This allows me to adjust the mirror angle (although in practice, I've now settled on the optimum angle to it's not particularly important to be able to adjust-- unless my setup changes).
Place the mirror on the floor under the table. Place the projector on the floor aimed at the mirror. Adjust the mirror angle, and the distance of the projector from the mirror. Project the image up onto the
bottom of the frosted glass.
The projected image is limited a bit in size, but it's still easily 24"x24" which is plenty of room for combat. It is also extremely bright and the colors are vibrant.
Someday I intend to have a dedicated game room (in a walk-up attic, for example) and even if I had room to project from overhead, I probably wouldn't to it at this point-- so happy am I with the results I am getting. The only thing I would change in this "dream" setup would be to get a bigger table surface and build a more permanent "Ultimate Game Table" to house the projector/mirror securely.
Of course by the time I have a game room like that, as I think I mentioned upthread, it will probably be easier for me to just buy a (cheap!) 48" flat widescreen LCD and mount that into an Ultimate Game Table.