I was thinking of a similar approach, but mounting /nix/store from the host into the guest will only work if you have a single guest.

For multiple guests, you should rely instead on: * A snapshot-able filesystem with the option to create clones (like ZFS). I think this is a great idea actually. * Exporting /nix/store via NFS, so you can have multiple writers (but this creates some tight coupling in that accidentally deleting stuff there may disrupt all guests).