If anyone is wondering, I figured out how to do this. There's a pin that runs from the inside of the cap, and goes through a little hole in the tube. You push the pin in with a punch, and unscrew. The pin may move, but it's actually the end of a fairly elaborate spring inside the cap, so just push away, and it'll still be there later. Just push, and unscrew. Obviously, watch how you unscrew the cap, as it will be wanting to fly of when the threads release.