I’ve got a prototype that should work. It will also resolve the chroot/pivot_root issues I’m having with nix-build from another issue reported. My main issue right now is trying to figure out the best way to manage FreeBSD compatibility / how to rebuild the credential handling such that it can work correctly and safely and in a compatible way.
Just keep an eye out in the mergerfs releases for a new release with whatever this may become. No timelines though.
Actually, if you’ve the ability and time to test out a branch: GitHub - trapexit/mergerfs at idmap
Basically gave up on the way I used to manage creds. Will have some subtle impacts that I’ll describe in final version’s docs but the branch is generally feature complete. Still need to explicitly set allow-idmap option.
Great to see more FUSE filesystems gaining support for this!
Hopefully @IdyllicHappiness is still around and can give it a try in his environment!
Absolutely, would be more than happy to!
@trapexit I’ll give it a go later this evening (UK time). I can see the branch but can’t see a case on GitHub, so unless you have a different preference, I’ll DM you your account here with my findings, as to not turn this thread into an ad hoc development discussion 
Cheers!
Edit: Have sent a private message on this platform.