The DBW throttle uses a pair of potentiometers (or rheostats, if you prefer). Gotta have two in case one fails.
You'd have to get into the ECU logic to be able to (easily) pick up on when the car is in-gear coasting, but not idling. Basically, you want the alternator to charge when DFCO is enabled. If you could come up with a way to pick up that trigger through the OBD and switch a relay with it, you'd be golden!