I bought a new Vectrix in 2007 and one of the nicest things about it was the throttle control. The twist grip was very intuitive in its operation. There was a neutral point that was spring loaded. Rolling the grip rearward worked like the normal throttle on the Zero. Release the grip back to it's neutral point and the scooter would coast with no regenerative braking. Roll the grip forward and you would start to apply regenerative braking. The more you turned it, the more braking you got. I usually used this means of stopping the scooter during normal driving. The Vectrix did not have a foot brake but had brake levers on each end of the handlebar which is common in scooters. On my Vectrix I swapped the brake levers so the front brake was on the left side and the rear on the right--opposite from the factory position. It was unnatural to try to control the rear braking effort with the twist grip while applying the front brake with the fingers of the same hand that was being used to control the rear brake. Once stopped the grip could be rotated forward and the scooter would reverse. It would not reverse until a complete stop had been made and the twist grip was returned to it's neutral position. I seem to recall a light on the dash warned you that you were in reverse. As noted in the original post, the people that hold the remnants of Vectrix still hold the patent. The fact that we haven't seen it on any other electric motorcycles/scooters means they either aren't interested in licensing it or they want a very high price for the license.
Even if we cannot get the throttle controlled regenerative braking on a Zero, It would be nice to have a hand operated brake lever on the "clutch" side of the handlebar in addition to the foot brake (or in lieu of it) similar to a scooter. I know it isn't motorcycle style to have hand brakes but with absence of a clutch lever or anything else to to with your left hand, it wouldn't hurt to have it there. If you look at the Pikes peak SR/F you will see it is fitted with a hand brake on the left handlebar. If its good enough for racing----well you get the point!