Stopping a Onewheel while fully in control of the balance of controls is easy enough, sure.

Having a Onewheel stop for you in any regard other than by the control of the rider is a guaranteed ticket to eating asphalt. Doesn't matter how slow you do it. People already fall off and eat shit on these things frequently when they're functioning properly, because the whole thing is operated by balance.

The wheel is instructed to move forward by the rider leaning forward. So you're tilted in at about a 60-70 degree angle from the ground moving at high speed. To stop, you lean backward, and the slowing of the wheel is counteracted by your shift in balance which drastically lowers your center of gravity as the board goes from pushing the rider to being pushed by the rider, as your angle shifts around to 110 degrees fully behind the board. Assuming rightward motion to illustrate the example, you go from standing being pointed like this [ / ] to being pointed like this [ \ ].

If the board slows speed without your consent you're going to fall off it. Guaranteed. It's a fact of physics. It could speed up without much issue because the balance of - well, balance, doesn't shift by much. But slowing it down shifts the balance across the vertical axis. Unless you also shift your body across this vertical axis, you're going to eat street.

Please stop, and just take the explanation from folks who have ridden these. I completely understand your point that if something is wrong with the device the smart thing is to prevent itself from being used. That makes sense. But you've now quadrupled down on this idea that will get people hurt. Just let it go.

That said though I'll also take my own advice, because it's not like designers at Onewheel are reading this thread looking for solutions. Nobody is in danger of thinking this plan is a good idea. So our discussion here doesn't really have a point anyway.