Neat idea, very similar to concept of running a chain through a pipe to prevent cutting, but in three dimensions. I do wonder what effect freezing would have on it though.
The grates that protect prison windows are often made of pipes with a bunch of loose rods inside. Cutting through the pipe itself is relatively easy, but the rods inside are unfastened, so they tend to roll under the cutting tools.
Edit: The principle is actually somewhat similar to the anti-drill protection on door locks - a small disc in front of the lock with a slit that the key goes through. The disc makes it hard to drill through the lock, because it spins together with the drill.
People do that? Also, how does it work?