<<The performance of vboxsf is horrible - that's why there are so many
recommendations to use NFS or CIFS for guests. So again I'm asking if
there's any issue with accessing the .fdb via NFS - given the exclusive
access I've described.

I don't see why there would be...other than ensuring that the access is
indeed exclusive.>>

Simply put.

As long as the database on the NFS mount cannot be accessed by another Firebird server
on another system then you can use this "feature" (RemoteFileOpenAbility).

Regards
Paul