Now, this is really just a thought experiment by a bored engineer, but I'd like some ideas as to whether this concept would work in practice.

What I have in mind is a single-wheeled bike trailer, fitted with its own brake and wheel hub motor. It would function in a similar way to the over-run brake on car trailers:the trailer hitch is telescopic, and compressing it actuates the brake, but also with tension on the hitch controlling the motor's throttle. That way, the trailer would effectively 'follow' the acceleration/braking of the bicycle without greatly affecting it.

I think this principle would allow you to safely haul heavier loads than with a conventional trailer, since you are no longer relying on the bicycle's brakes for stopping, and saving the cyclist's legs a bit. In theory at least, multiple trailers could be coupled together as a road train, though this might lead to forward/backward oscillations as the trailers tried to follow each other. I'm surprised over-run brakes haven't really been done before though.