I fear welding across a door skin, no matter how good you are. Metal warping/oil canning for me would be a sure thing. Then it gets ugly fast. I agree that Bondo (or any other plastic filler brand) should not be used to fill pinholes. Plastic filler absorbs moisture and will cause additional rust in time...especially on a door skin that sees moisture every time it rains or you wash the truck.
Never thought of using lead, but it should work ok, I guess. Just remember that the metal around the pinhole is very thin at this point and will remain so even after leading (assuming you lead from the outside and the rust was from the inside).
Good luck.
