This is exactly what I was thinking about doing as I have a Megasquirt, a Raspberry Pi, and an Arduino all three in my car. The Arduino is set up to run a small column mounted display and the Pi is more like an infotainment system that runs the tuning software, navigation, and media. I'll be really curious to see if you can figure out the protocol to talk to the Prius EPAS unit as that is likely the direction I would go if I ever get around to doing this to my car.