I don't think your initial assertion is correct and even if it is nothing requires you to use such a lock.
Most high security locks can be re-keyed. There's no need for it to be keyed to any existing master. I consider that better than "one of these 6 predefined TSA keys that can be 3d printed oh and also it is labelled."
Nor does the threat model of "dedicated attacker" make a lot of sense. If that is your constraint then flying with complete guns is probably not for you.
There are easier ways for criminals to get guns than a mole in the TSA or baggage handling. Like, stores. A decent padlock can secure a gun case beyond the "crime of opportunity" level required for protection in a secured environment like an airport.
If you're really paranoid you can even build in trap pins so attempting picking or using the wrong key can permanently bind the lock. The point is you are in full control of the lock and don't have a government mandated, well understood backdoor included.
Finally, all of this misses the point which is that you can put a firearm such as a starting pistol, flare gun, frame or lower [1] in your luggage and use whatever lock you want. This means you can protect the rest of your stuff from getting swiped by TSA or baggage handlers.
[1]: https://www.atf.gov/firearms/firearms-guides-importation-ver...