Actually, according to the United website, it says that if you use a TSA lock, it can speed up the check in procedure. I've never seen anything on the TSA or the airline's sites to say that you cannot use a TSA lock on it. On my normal plastic gun case, I use a pretty heavy lock around the handle. It's debatable whether good lock on a piece of plastic is more secure than a crappy lock on a piece metal. When I get back home, I'll redesign it to allow for a thicker hasp made from a flat piece metal so that a lock with a thicker shackle can be used on it.
The main thing though is that this makes for a pretty compact container to put inside of your luggage that will also easily accommodate 2 handguns plus a few loaded mags if you add proper padding for everything.