Here's what works for me:
Notice that the two shiny barrels touch and complete the circuit when the brake calipers move-in. The rubber boots on the ends of the barrels absorb any "slop", and allow full braking force to be applied. I just disected the stock boot, and removed enough of the boot to accomdate the barrels. I'll bet that any geometry teacher would appreciate the inevitability or reliability that two cylinders would provide for colliding and completing a circuit. They basically can't miss each other. The barrels are insulated from the brake cable with aquarium air hose.
I looked into a solution like Mr. Larkin's door interlock switch earlier this summer, but concluded that the introduction of the interface for the switch to the bike, was convoluted and prone to unreliablity.
Has worked all summer on my prototype build, and plan to continue using it for build #2 (his and hers 150 MPG moped set). Switching-over to a Honda Magna for next summer.
Still working on my anti-theft stun electrode seat stinger, but watch for it next spring.
Just keeping it simple and stupid. Works for me.
If it doesn't kill me, it makes me stronger.