[The_Techie_Stirlingite]

Freebeard, this is just something I was going to test and see if it's feasible. No plans for commercialization.

As far as the RasPi goes, I have 3 RasPis and honestly think they would be MASSIVE overkill.
One does not need a 1-GHz CPU, a graphical OS, or Wi-Fi to run a motor controller and interface with a vehicle. In this application the Raspi would be more trouble than it is worth, having to load an entire OS before the hybrid system would work.

With an Arduino, the code loads quickly at power-on of the vehicle ignition, and the system would be able to run a quick self-check (blipping the assist while simultaneously watching engine RPM to check the motor/controller functioning) before the 12V starter kicks off the engine. The system ECU may not even have to shutdown when the car was off, since Arduino draws so little energy.
By contrast, the ARM CPU of the RasPi is a power monster that would drain the 12V battery quite rapidly with the ignition off. It would have to shutdown completely, and then startup again (10-20 seconds bootup time.)

As for a UI, the hybrid ECU could run a Scangauge-esque character LCD showing the vitals of the hybrid system (an idle-stop indicator, "charge <-----> assist" bars, hybrid battery state-of-charge, etc.)