Though your chances of picking up a F&S derailleur system are probably slim, I bet it wouldn't be that hard to fab one up... basically you'd have to attach the business end of a simple chain tensioner to a derailleur claw... friction shifter on the top tube... Interesting project!
Yeah, when you brake the chain slack moves to the top part of the chain, and the bottom part goes taught. There's a hook over the chain ring to keep the chain from falling off entirely; you can see it on the Copake bike. Obviously you'd want to have as little chain slack as possible....
That sure would be a fun project and I really might attempt it.
Here's that hook that keeps the chain from popping off the chainring
