I strongly suspect that Rick's motor is running in open loop all the time to get mileage numbers that low. The HAC sensor is something I didn't think of; it is mounted externally from the ECU on pre-89 Supras...mine is below the glovebox and 1.5' of wires connect it to the ECU.

Consider this, I think the NA 7M has 305 cc injectors. I have burn through nearly 3/4 of a tank of gas at the track where my car is either idling for hours or at WOT. My turbo injectors are nearly twice the size at 550 cc and my lowest mileage has been 14 mpg. Have you smogged the 4Runner with this engine or checked the A/F ratio with a wideband O2 meter?