In all my years I had never heard of ferrotyping fluid. I just got my hands on a bunch of ferrotype tins in pristine condition. Years ago I would soak my prints in a little photo-flo and slap them on the tin and roll them out, and all was well.
Hardener makes it impossible to wash the hypo out? That's one I surely never heard. I'm not taking issue with your statement, but I am scratching my head trying to figure out why. I always figured the washing process was simply diluting out the hypo from the backside of the print until it was eventually diluted away. Trying to wrap my brain around how a harder, scratch protected emulsion would stop that.