I'm sure you could put an auto-activated brake light together relatively simply and cheaply using a mercury tilt switch. It might need some careful calibration so it doesn't activate on steep downhills, though,
I worry about brake lights on bicycles. Maybe an affordable system will be designed and put on the market.
Maybe a few people will fit them. Some will fit them properly. Batteries will expire. Bulbs/LEDs will go phut.
Cycle Brake Lights will attract some media attention. Then someone without brake lights will get rear-ended...
And the driver might say
"I didn't see any brake lights". And the media might say
"The rider had no brake lights".
The
heavy-heavy anti-helmet brigade already have enough to be cross about. This might make one or two explode.
No lights illuminate at the rear of my car when I throttle off. People seem to know when I'm slowing.
I'd leave things as they are and (in line with Mr Paul's comments above), I'd try to stay calm and polite if I did get bumped.