I tried this a few years ago.. I used a POT to modify the Air Intake temp. In theory it was supposed to make the truck think it was warmer so it would lean the mixture (air is less dense when hot)..
I just noticed you said ECT.. thats the engine coolant temp... I don't know how modifying what the computer saw there would help..
~Mark
edit: I forgot to say I never saw any difference in mileage... I think it was because it pulls the timing back some when its hot so you acutally loose a little power