Yeah, the emulator products are priced so they might make sense if you desperately need to keep some dedicated industrial computer running. (You'll find MFM or RLL hard disks floating around in the strangest places, up to and including aircraft avionics systems.) For a hobbyist? Not so much.

I know it's a long shot, but... do any of the contacts you've found that used to work on the GCC Hyperdrive have access to the source code for the drivers and ROMs? Far more practical at a hobbyist level than creating an MFM hard drive emulator (where you have to deal with high speed serial data streams with no flow control) would be creating a complete "Hyperdrive Substitute" consisting of a Compact Flash/PATA IDE port on a 68k socket daughter card. The hardware is trivial. (Homebrew devices like that are a dime a dozen in the Atari ST and Amiga fields.) Writing drivers for custom devices is always the hard part for the antique Mac community. Since IDE/Compact Flash drives actually (partially) emulate the programming interface of a Western Digital ST-506 controller chip having the source for the Hyperdrive drivers would give you an easy leg up for writing a driver for an Atari ST-style parasite IDE board. A good programmer could probably adapt them in a few hours.