I'm not sure how exploitable a read-only virtiofs share is, so this is perhaps not appropriate in some circumstances.
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).
The problem with that is that the VM can see all the needless software, so if your goal is isolation, having a smaller closure is much better from a security point of view: if there's no coreutils bash etc then there's no risk of getting a shell spawned by an attack...