One of our members, Jackitup (Jon), routinely bakes his cracker-style pizzas on a sheet of aluminum foil on top of a stone (for example, see Reply 66 at http://www.pizzamaking.com/forum/index.php/topic,5762.msg49746.html#msg49746
). I believe he uses that method to keep his stone in pristine condition. I would think that moisture or steam would develop between the foil and the bottom crust during baking and result in a less crispy crust. Does that make sense? Jon has a convection oven and he oils (the bottom) and pre-bakes his crusts, so maybe his crusts dry out enough to retain their crispy character. He has also been able to load up his pizzas with lots of different toppings without any apparent problem.