Hi! In your original post you didn’t say your users’s directories already exist, it sounded like you wanted to create a (sub)volume with custom UID on demand.
If the user’s directory already exists then you can run your server as a different UID/GID, see for example
It’s a bit out of date but the idea still holds, create a pre_spawn_start hook that passes the uid/gid to the singleuser server, run the server as root, and switch to that UID. You might even be able to run as that UID instead of root but I’ve never tried it.