I wonder if this modification could at least be made harder by having some kind of heartbeat. Build in a microphone whose sole purpose is to receive inaudible, high frequency impulses (maybe a hash signal?) from the speaker every once in a while. If the signal does not arrive in time, the AirTag shuts off.

Of course, no measure can beat physical access to the device but you could at least make the tampering about as expensive as building your own device from scratch.