BLDC motor emergency stop
This type of system is fairly common but you must remember that this is a reactive system. It will not HOLD the vehicle but rather, will retard it's speed.

That is, the amount of brake force generated will depend on the speed of the vehicle. When stopped, there is ZERO brake force.

A vehicle parked on a steep slope will continue to run down the hill, all be it at a much slower and fixed rate. As such this mechanism can in no way be called an "emergency STOP" system.

ADDITION:

Current limiting would also be prudent to prevent the motor from burning out. This would be especially a problem if the vehicle is on a really steep slope. At higher speeds this would of course cause braking to be reduced. At some point, maximum braking would be achieved as the vehicle slowed.

ADDITION 2

For emergency stop though it should be considered supplemental braking at best. Some sort of mechanical braking system capable of stopping the thing with or without the assist would still be a MUST-HAVE. In the end you need to decide if the extra cost and reliability reduction of the effort is justified.