Dodgy Geezer,
You mentioned that the point of wanting this braking device is for use in an emergency stop. I would also suggest you look at some kind of interlock to prevent an unwanted start of the spindle. In the most serious accident scenarios, including an unexpected start during a manual tool change, the spindle would have already caused serious injury before you ever managed to hit the ES button. Just thought I'd throw one more worm into the can!

Kit