How long are you pre-heating? I don't understand why you need a big fire? I understand the issue with the overly conductive floor, but once you're off the floor, you're off the floor. How hot are the walls farthest from the fire when the deck is between 800F and 850F? For a 60 second pie, I think you will be forced to bake the last 40-45 seconds in the air, but I think you can get there with the right fire management.
How would you thermally bond the board to the existing floor? If you get an air-gap between the two, you might end up at the other extreme where you don't have enough heat below to balance the heat above.