I think this is a solution looking for a problem.

Let's just suppose one has a J3 Cub or Luscombe without an electrical system, and the owner installs a navigation aid that is electrically based, such as a radio, transponder, or ADSB out hooked to a battery. Heck, let's throw in a starter that works only by battery on a total loss basis.

Now let's suppose said owner/pilot turns on the radio/transponder/ADSB before entering airspace that require them. The pilot is happy. The FAA is happy. Everyone is happy.

After leaving the airspace that require such things, suppose the pilot then turns them off to preserve the battery. The pilot is happy. The FAA couldn't care less. Black FAA helicopters are not going to chase down a J3 Cub to ask why the ADSB went offline; it's not required, and they have better things to do.

Yes, the matter should be settled in the regulations, but I wonder if the risk of stupidity on the matter is worth it.