The IAC adjustments are based on the following ECU inputs: TPS, CPS, CTS, VSS, and various switch conditions (brake, park/neutral, air conditioning, power steering).
I was going to disconect the TPS and let the Megasquirt power it, hoping for the most accurate signal, but I think that's a bad idea. I'm not sure how much the IAC function would be affected but I don't want to find out!