Has anyone ever looked into finding a reverse gearbox thru an industrial supplier such as Grainger or McMaster Carr? It just looks like something that would be used in automated production lines or something along those lines..
Is it just a planetary with a slip gear or is there something more to them?

Pondering the "bike" details, Fletch.

I can't imagine that something as specific as a reverse box would be made for any other purpose, I guess the internals possibly might be used in other applications but I doubt they'd need to cope with as much torque/rotational speed as they would in a drivetrain. Even if there were gearsets made for other purposes that fitted the bill, I highly doubt you'd get something suitable out of the box including the casing (to fit in a tunnel), output flanges etc etc.
Best solution = electric / push

Chris