Not purely in-memory, but something like https://github.com/jrwwallis/qcow2fuse https://github.com/jrwwallis/qcow2fuse maybe? It's clunky compared to OSX's DMGs

Not purely in-memory, but something like https://github.com/jrwwallis/qcow2fuse https://github.com/jrwwallis/qcow2fuse maybe? It's clunky compared to OSX's DMGs, but if you squint it achieves similar ends. Otherwise you could achieve this with a tmpfs wrapped to serialize to a tarball (preserving symlinks) when unmounted.