Regenerative braking for braking assist
Dynamic brakes are used on electric trains (sometime true regenerative brakes are not used because dynamic brakes are far simpler). I think they may also be used on trucks to regulate speed on long downhill roads.
You’d need a generator built into a wheel, like a dynamo hub, but somewhat larger to provide greater braking force. Power is dissipated in large resistors with cooling fins much like the cooler on an PC CPU. Depending on how sophisticated you want to be you could include a controller circuit to vary the braking force.
The issue is that all this adds significant mass to the bicycle, and the generator will create a non-zero resistance even when off (unless you provide a clutch to disengage it). Given that by definition you need to ride this bike up a hill in order to descend it, is the trade off worth it?