E-bike levers have electric switches in them, so put that to a red LED security floodlight via a battery, and your brake light needs are sorted. I have a few spare for postage cost. One lever, bit of wire, and a red battery lamp, job done in ten mins.

However, with the necessity issue, i just don't think a working brake light will translate. Nobody expects a bicycle to have a brake light, they will just see a red light. With something wrong with it.
A flashing red light is a brake/fog/tail light in one. A motorist knows there's a bike there. It's not going so fast, they're gaining on it, it's a hazard. With a brake light, it won't matter if the cyclist is braking or not, as they're still gonna be getting in the way anyway.

Yes it's easy to make a brake light. So without there even being a challenge/project in it, i really don't see the point.