Just a thought...
Why not just hack a mini-keyboard that has the Suspend button, and plug this keyboard into the IOpener? It should be trivial to tap into the keyboard's suspend-button switch. Once that's done, a simple circuit could be made to momentarily close the suspend-button's switch when the AC circuit loses power.
With this setup, the IOpener can be constantly connected to the car's battery The keyboard hack will auto-suspend the machine when you turn the car off; and since you've already got the BIOS to start when it sees power, the machine will auto-resume when you turn the car on. About the only downside is the extra wires running into the modified keyboard.