I think that there is no way to fasten a disc brake on those bikes with 'back-pedalling' brake...
How about using the horn button for regen brake?
An advanced idea:
When one pedal forwards, the upper portion of the chain are taut, when breaking by pedalling backwards, the lower potion of the chain are taut; mounting a small spring-loaded sprocket to the lower potion of the chain with a switch that activates when the (lower portion of the) chain are tight, activates regen brakes...
Or just make it the fact of biking that particular bike that pressing the front brake lightly wil activate the regen brake, and if one want rear brake; pedal backwads...
Edit:
My mother also wants a front wheel motor, and she also have a bike with internal 7-gear and back-pedalling for braking... I'll see how we choose to solve that one...