You could mount a DSK file (even FAT12) with OSFMount and there you have your folder. Use the DSK image in OpenMSX. Or isn't this what you meant?
What about implementing a virtual HDD interface/"cartridge" for this? The ROM in the cartridge could contain DOS2 + FAT16 support. DOS2 file-level routines can be routed directly to host OS disk routines, low-level disk routines can be stubbed. Then you do not need to work around or even bother with the peculiarities of existing HDD interfaces.
openmsx -ext dirashdd ~/msx/my_msx_hdd
OMG! That is the most qualified disqualification of a proposal I've ever read!
Hahah, it was especially for you! IRC?
'the most qualified disqualification'
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