It would seem that if you have a lightweight cast with a metal frame of some sort--a friend of mine currently has one for a combo sprained ankle / broken leg injury--that you could conceal a blade in it and get past security.

You'll set off the WTMD and/or have to be wanded, but the metal frame that you can claim cannot be removed would set off the wand and have to be cleared eventually (they can't deny you flying over a cast you cannot remove without potentially causing medical injury, after all)... and since you're not planning to carry on explosives, you won't set off the swab.

Of course, what you could actually do with said blade is pretty limited, but it goes to show one more way a would-be bad guy could get a weapon onboard without being caught.