The only issue I could think of it that the pins wouldn't line up at all so you would need to do a lot of routing to get it to connect right on the proto shield. Also on MY protoshield, the top sockets do not line up with the arduino. You will need to make sure that the top pins on one line up with the bottom pins on the other. They do line up on my ethernet shield, so the protoshield could go on top. But they're all different so you'll just want to be sure.