Well, a valet switch on the line going to the aCC terminal will allow you to power it on and off while driving. It won't give you the ability to run the PC with the ignition off, but a simple workaround would be to leave your key in the ignition. :|
What you're looking for \requires some rudementary logic. You might be able to pull something off with a dual-switch or relay setup, but I'm not certain how to pull it off. Someone else might.