Jon,
So you are going to place the metal pizza pan inside your water pan, and then not use foil? The air gap shoudl provide the insulation so that great doesn't burn, but if you were cooking multiple butts it might overflow. Also, if grease dropped down where the two pans meat, it will probably leak through and get to the bottom of the water pan, where it is going to burn.
By the foil from a restaurant wholesale if you want to save some money, it gets expensive in the small rolls.