I'd be interested to see how Airport Security likes it, but it's not a bad idea for a last-ditch, take-anywhere backup weapon. I sure as heck wouldn't want it to be my primary defensive tool, if possible.
I wonder if the TSA would let you carry a pool ball in a sock? Tell them it's your lucky 8-ball, and the sock is to keep it from getting scratched?
The "combat pen" is an interesting concept, let us know what you think if you decide to order one. I'd seriously consider building one.
Peace,
Pete Zaria.