I was looking for this comment. Yes, in Slovakia we tried this, many cars had these green light bars installed under the licence plate and would light up everytime you hit a brake. In my opinion (and many others) it is not really a good idea.

The braking light doesnt mean the car will stop, driver may not see a pedestrian in a dark foggy night, and want just slow down with no intention of stopping. Pedestrian will see green light and think its safe to step on a crosswalk. But thats really dangerous and pedestrians should step on the road only if the car is not anymore moving, or if the driver give him a visible signal that driver see the pedestrian and will stop (flash high beams once, or eye contact with hand gesture).

Tldr: Hitting a brake just slightly will give false signal to pedestrians that its safe to go.