Would this setup require you to configure and mount a derailleur hanger in a new position,
forward of the cassette? I suppose it wouldn't matter what position the derailleur ended up, bottom pulley up so to speak or upside down from it's standard operating position, but there might be interference from the seatstay or rear brake assembly.
I just tried a spare derailleur in that forward position on a large frame, hardtail mtb and there would be clearance problems with the cantilever brakes it has.