As some of you predicted, this did not work out very well.
The first issue was that the heat up time was terribly slow. I build the fire on the side of the oven in order to keep the woodpile off the soapstone. Slowed things down a LOT.
So, giving up on that as a concept I would accept, I moved the fire over the stone and more pervasively in the oven. As I expected, the heat up accelerated.
The second issue was the pie bake. I had to move the pie off the floor super fast to prevent if very burning too much. Spent way too much effort fooling around with doming and protecting the bottom. And, they blackened a bit too much anyway.
Third, the stone looks like it cracked. I will be able to check more closely sometime tomorrow when it is cooler and I can remove it with oven gloves.
So, although the learning experience may have been worthwhile, cooking this way was not worthwhile.
I may, at some point, try out firebricks. For right now, I will go back to using the oven as it was designed to be used without a modification.