I saw somebody in a white van slow down and eyeball the Jazz yesterday, so I feel your angst. I must admit, I've given some thought to devising some sort of alarm, rather than a guard. It seems to me that it's only a matter of time before they start coming equipped to remove the common guards. The good thing about a guard is that it should slow them down, and hopefully they'll make more noise.
A tilt sensor isn't straightforward because it has to be able to cope with parking on a slope, and if it's made to accommodate that, it may be defeated by the slow tilting action of jacking. I was working on the lines of an attached cable which would trigger an alarm if pulled or cut. One of many projects, unfortunately.