Bicycle Flywheel Trailer

They do make trailers for bicycles, and they generally have two wheels. So, it should be possible to put a flywheel (assume TwinFlywheel, per link) system in a bicycle trailer.

And, some things can be simplified, because the bicycle itself doesn't need much modification beyond adding the trailer hitch. The trailer itself can have the electric motor/generators in its own wheels, and can push the bicycle. An extra cable that reaches to the bike handlebars can allow the rider to activate regenerative braking on the trailer (rider doesn't use the bicycle brakes except in emergency), when the cable isn't acting as a throttle.