What would be a good material for a portable video projection surface?
Maybe showerboard? I've used it in the past as a dry erase board. It comes in large sheets which you could attach to the wall via screws or hangers of some sort. A dab of white paint over the screw holes and you should be ok.

I don't remember exactly how expensive it was but I don't think it was more than $20 per sheet ... and they were fairly large sheets.