But in the case of the PIA program, the hex address that's broadcast in the ADS-B packet is unpublished, not associated with the tail number of the plane anywhe

But in the case of the PIA program, the hex address that's broadcast in the ADS-B packet is unpublished, not associated with the tail number of the plane anywhere except in the FAA's internal database. It can be a very effective way of obscuring aircraft identity (though see the DEFCON youtube link I posted for potential vulnerabilities).