Not sure if the Honda's have it, but the Ford and GM's I've worked on have what they call "retained accessory power" (RAP). This is the function that allows the radio, power windows, etc. to work after the vehicle is shut off and before the door is opened. May not be exactly what you are looking for, but might be close enough.
Finding a circuit that has power when this function is active might be easier. I've got a Chevy Colorado truck at work and I found the lead that would have been used for the sun roof (if it had it). I was able to take that circuit, which was taped off in a harness inside the "A" pillar, and use that. Putting a fuse in the "sun roof" slot of the fuse box provided the power I needed.
I used the feed for the ignition sense lead for a couple of Kenwood VHF and 800MHz radios.
I'm sure as an amateur you know the issues with tapping into existing wiring, specifically noise and current capacity, so I won't go into that.
I would recommend pulling off the battery directly and using a relay switched by the RAP power.
Other option is to get something like a Lind timer: 2-hour Shut Down Timer (non-fused with screw terminal connections) | Lind Electronics
You can often find them (or similar models) on e-Bay. Tap the trigger line into the ignition and the power directly off the battery. Might cost a bit more, but will be more reliable and likely less noise issues.