Hi BOB!
I honestly thought this out for a little bit. I thought about rigging switches up to V brakes, and to the levers. It is all just too impractical. Unless you can do molding or machining. Some do have there hands on some good tools. But even then, it would be hard long job.
The heat shrink is not a good idea either, it would have to be tight, so tight it may end up activated permanently, or otherwise it may move around until it does get tight and activate when not in use or intermittently. NOT!
How much pressure will it need to activate in time?
Is the rider going have enough pressure on it 100% every time. There are too many variables in other ideas for this important function to be considered reliable.
The idea of the brake switch is
1: For safety, Cuts the motor off if there is an error in the throttle or controller.
and
2: To stop the user from operating the motor and brakes at the same time.
Operating the brake and motor at the same time can put much stress on the battery and place undue stress on the motor and most importantly place undue stress on the forks and dropouts. It has to be pretty tight timing. The front brakes as you know is the most important. So the front brake and its switch could be best located on the throttle side, as the user often release the throttle to brake, some times a little throttle remains as the base of the thumb can cause the throttle not to back off 100%. In some controllers there is a small delay when brake and switch for the controller to deactivate the motor and give regen. letting off that throttle just a bit when the rider reaches out for the brake lever in my mind makes a big difference to the forces that may remain from the motor due to any delay that may occur in the controller.
I honestly feel it would be wise just to get a new shifter. The reasons are important that the brake switch reacts at the first touch moving the brake lever.
What I tell you next depends on whether or not you have the front hub or the rear,
If you have a rear hub you can use the front brake to operate the rear brake regen so that the stopping power v brake/regen is placed evenly on both wheels. Another reason I like rear hubs.
On the front hub the rear brake can be on the throttle side so that regen and v brakes are not pounding at your forks at the same time.
Both strong rear braking and front regen braking isn't too bad to have rely on the throttle side. Get used to using the throttle side brakes so you do release that throttle just a little.
Can you do us a favor?. Can you test the brake switch while you hold the throttle on and tell me if there is some delay in the brake switch deactivating the motor and if there is a delay estimate how long the brake switch takes to deactivate the motor.. Even if its a 1/4 of a second I need to know.
Then we may decide what the No.1 best advice to give all concerning the newer controllers.]
Thank you.