You are correct in that cast iron pans won't let the bottom heat quickly enough so that the entire pie, bottom to top, is finished at the same time.
I used to use cast iron pans before buying the proper pans. What I would do is when I put the cast iron pan in the oven to bake the pie, I'd put a large burner on the stove top on medium at the same time so that it would warm up. Then when the top of the pie was where it was where I wanted it, I'd pull it from the oven and put it on the burner to finish the bottom. That typically took 1-3 more minutes. You'll need to check it every minute or so using a spatula to lift the pie to check bottom doneness. Let it continue cooking the bottom of there to where you want it, then remove from the pan. I would get very good results, as the top of the pie pretty much stops baking when pulled from the oven, and the bottom finishes as desired. It's a good work around for occasional pie baking. However, nothing will beat a proper pan for convenience, consistency, and ease of baking if you're making them often, or need to make more than one at the same time.
I've never personally tried cast iron pans, but I can't see them working well. The heat needs to reach the pizza fairly quickly, and cast iron is fairly thick and dense. it'd probably take a good deal of time for the heat to penetrate the cast iron and get to the dough.