It's not a big job to wire the detector you're talking about in your first sentence - it's more or less a relay and a capacitor (albeit a big capacitor, or maybe a battery if you put some more wiring and a charger in the circuit)

For the second one... you'd need to maybe consider tapping into the CAN bus and listening for a door unlock event, then powering up off that, and/or stripping everything out of the kernel and services that isn't needed... you could probably get that boot down to 10-15 seconds