Stupid question, but relevant:
Are you simply after an ND emulation for a limited number of platforms?
If so, it might be far simpler to program each platform specifically.
This is the way it's done with the inexpensive Chinese floppy
emulators and they work quite well for their purpose. One popular one
uses a STM32 ARM and little else. The larger benefit is that the flash
drive used to hold the data in its native form (i.e. not MFM pulses).
One variation I've seen even uses a USB connection to change the program.
I'm guessing that the primary consumer of this device will be the DEC
community. The personal computer community has had various devices
(such as IDE or flash drive) as replacements.
--Chuck