Mounting it would be the tough part, most of those bar-end mirrors have a ball-and-socket type mounting/adjustment point. So you're essentially trying to find a way to stick the ball part onto your car.
But, by the way of good news, there is no legal standard for optics on car mirrors that I'm aware of, certainly not in California anyway
Our state vehicle code just says that there must be 2 mirrors and they must afford a view of at least 200' to the rear of the vehicle.
Your state laws may vary. (YSMV?)