Clearly, they're working on a wide variety of different mechanisms, and most of them will go no further than the lab. The mechanisms I like best are the ones that go inside the cockpit. Pilots already can change their transponder code to (quietly) indicate "I'm being hijacked". Take that to the next level, and allow for remote controls, better authentication, and so forth. The intriguing question, of course, is whether the remote control interface, itself, is a new vulnerability of significant concern.