Some years ago, during a career day at my high school, a local studio photographer used an overhead to show some of his 8 x 10 color transparencies. It worked fine and, needless to say, the kids were surprised to learn that he wasn't showing enlargements. Actually, I was surprised too, since I didn't realize anyone in the area was using 8 x 10!
Back in 1970's, I tried enlarging a few negatives onto what I think was called Kodak Fine Grain Positive. The stuff could be bought from Freestyle in rolls about nine or ten inches wide and processed under safelight like prints. It worked fine as an overhead transparency as long as the density was kept a little on the light side. It was hard to deal with, however, since it had a pronounced curl and had to be taped to the projector or mounted in a strong frame. I've looked for this film in recent years and had no luck finding any.