A flaw I see is that hand brakes can require considerable force, available from your grip, whereas the shift levers are made to adjust things, not to apply large amounts of force.
I've seen photos of a number of oriental pedicabs that had what appeared to be large homemade sure-fire brakes. You might take a look at them. Probably not the route you'd want to take, but it's been done.
__________________
"be careful this rando stuff is addictive and dan's the 'pusher'."