I'd check with the TSA before locking a cue case, even if it is inside checked luggage. Since I think your luggage has to be unlocked (the TSA apparently has the right to search even checked luggage and can break or cut a lock to get in if they want), I would assume they would also have the right to get into anything locked inside your luggage. I'd hate to have a nice cue case broken by the TSA because it was locked and they decided to force it open.
I don't travel with a custom or expensive cue anyway. I've got a lower end cue for that purpose. Besides being damaged by the airline I wouldn't want an expensive cue to get lost in transit, stolen out of my hotel room, or have to leave it in the hot trunk of a rental car all day long while I'm at work.
Of course I am not a professional so I don't need to take my best equipment with me everywhere I go. When I travel (usually business) pool is just a recreational thing for me to do in the evenings to unwind. If I happen to find a match while on the road it is usually for friendly stakes.
If you do need to travel with an expensive cue, one option that I think some guys use is to ship it to your destination via UPS, Fedex or USPS. This way you can insure it for whatever value you need to and if you mark it fragile/do not bend etc. you have a better chance they will treat it properly than when the primates behind the wall at the airport get a hold of it. And you don't have to compromise on protection because it has to fit inside your suitcase. Plus, now that the airlines are charging for checked luggage (which BTW, is the dumbest idea they have come up with in a long time), it might even save you a little money and it will avoid baggage claim at your destination if you would otherwise be able to carry-on were it not for having to check your cue.
Hope this helps.