Keeping it powered on
It's very simple. I have such a system in my car. Radio amateurs around the world have APRS, which is basically the same technology as ADS-B. GPS receiver, controller, and radio transmitter. $100 per unit.
All I need to do is put a jumper wire in my trunk (boot) to keep it powered up by the rear aux battery and it will be on H24 sending out my position.
A few watts and a local polymer fuse is all you need, so danger of fire if not associated with a CB in the cockpit or EB is insignificant.