This came up on Discord recently—new member @calvincomma is working on a port of the Darwin vnd driver to NeXTSTEP 3.x, which is what allows mounting of files as disks.
GitHub repo: https://github.com/NattyNarwhal/vnd-nextstep
The project is not yet finished, and is not yet in a working state, but he is open to anyone interested in collaborating or contributing to the work. (He also notes he'll be away from his machine for the next week, so won't be able to reply to any inquiries until then.)