There is only one way the rapeist will probably find out that the device is in place, and buy then, it's already done it's work. And once the device has done it's work, the rapeist will no longer be in the mood for any further rapeing, and will be too preoccupied with pain the think about a beating.
I think Jock made an interesting point about the neibours. The best defence we have is to all be virulent and take care of each other. Another good reason to learn MA.