There is something about a turning motion causing precessive motion and self tightening a thread even if the bearings are free.
A planet where I can easily lock a wheel using one finger. Any reason why you think it would be a bad idea?
Couldn’t shift gears at the same time
Already a lever in the thumb position, would be awkward to fit in there
Harder to push with sufficient force – I think a thumb would get tired and become sore a lot faster than a hand grip.
Not sure you can lock a wheel with only one finger on a good surface when going downhill.. I can’t usually, and I’m not known for having particularly week hands or poorly set up brakes. OTB is more likely than locking up.
But it would also be easy to knock something together out of Hope parts if you knew someone with access to CAD and CNC.