it wouldnt be that hard to do with a couple relays, you could even use mosfets i think and go that route. what youre asking basically is a "latching" 12v signal. which means "turn the ignition signal on, and reset yourself once the car is started".
i do it with my remote car starter. you can set one of the auxiliary outputs to come one with the keychain button, and then when you turn on the actual ignition the pc stays on and the aux resets itself so its ready to be used again later. but two bosch relays can pull off the same trick if you dont have a remote aux to use.