Isn’t that what DriverKit can be used for?
https://support.apple.com/en-gb/guide/security/secd0a47c14c:
“DriverKit is the framework that allows developers to create device drivers that the user installs on their Mac. Drivers built with DriverKit run in user space, rather than as kernel extensions, for improved system security and stability. This makes for easier installation and increases the stability and security of macOS“
There needs to be some OS-side FUSE-like kernel interface for userspace drivers like this to be able to actually mount them in the VFS.
They're more geared towards Dropbox-like synchronizing use cases. Some developers have made it work for SSHFS too, but even that's already a bit of a stretch; for more custom solutions it definitely falls short: https://developer.apple.com/forums/thread/681325
The only option currently seems to be to implement a network file server such as SMB (which is what Google Drive used to do before File Providers existed!) or NFS, for which there even is a FUSE bridge available: https://github.com/macos-fuse-t/fuse-t
I was hopeful that Ganesha NFS [1] would suffice as a bridge for the various FUSE filesystems I need to use with macOS, but alas, they have abandoned the FUSE interface and moved to FSAL/FSAL_VFS [2].
It would be really nice to have a clear path to building a copy of Ganesha NFS that supported a whole host of the great FUSE filesystems out there built to this API.
[1] https://github.com/nfs-ganesha/nfs-ganesha [2] https://github.com/nfs-ganesha/nfs-ganesha/issues/759
If you've ever had a suspicion about files on an SMB share going missing, occasionally, when there's been a Mac client using the share, this does indeed happen.