You're going to start crunching numbers? If I say 700ish and it turns out that 750 is needed, is that going to make me dead wrong? There's going to be a number that compensates for the drop in thermal mass. Do I have that number? No, but it exists.
The reason why you've never seen any iron baked undercrusts of note is that, to put it bluntly, no members with any kind of skills have attempted it. Cast iron has, for the most part, always been a beginners thing- someone that saw Heston Blumenthal or read Kenji's article- newcomers that barely even comprehend pizza, no less NP pizza.
When I said I'd put money on it, it wasn't hyperbole. Pick a dollar amount (the higher the better
), find 10 undercrust photos that you consider to be flawless to be benchmarks. In two months, one of those benchmarks will be achieved.