[Occasionally6]

I have seen quite a few plug and play devices that intercept the pedal position sensors and change the response of DBW throttles. They are aimed at increasing the throttle movement though, the opposite of what is wanted (for economy driving).

One guy did a write up of one such device (in what he called a "White Paper") on it. If I recall it was in a Mercedes-Benz but they'll all operate similarly. His conclusion was that it was likely using Op-Amps to change the two (some do use three) voltage signals out of the pedal position sensors.

P-hack's suggestion of using a microcontroller seems reasonable, if the ECU mapping can't be accessed. The failure modes include dropping the throttle back to idle position if the brake pedal is applied so it won't be particularly dangerous if it did fail.