Sounds like a good RC-Cam project!
I had thought about creating such a gadget for the e-flight folks, but they did not seem to be excited by the weight gains from a tiny circuit (every gram counts to them). If my schedule allows it, I may just find a reason to do this for our camera app.
I wonder if some of the battery packs made for camcorders and digi-cams have the circuit built in.
Perhaps, but I imagine that many cameras have shifted the low voltage cutoff responsibilty to the camera itself. If the pack's internal protection board has a low voltage cutoff, then you may have to sacrifice a battery to find out.