It seems prudent to first mention this issue and then this aptly-named edit which seems related and has made hh_server refuse to run on NFS file systems. I am not very familar with file systems and have never touched OCaml before, so in trying to accomplish the question title, I have tried editing what I know: /etc/hh.conf and /etc/hhvm/{php, server}.ini, adding hhvm.[server.]enable_on_nfs = true by pure guesswork. No dice.
As I understand it from the issue, the change stems from the hh_server daemon being unable to register changes to the files via inotify on NFS drives, which is totally understandable. However, my VirtualBox is purely a test server for me familiarizing myself with Hack (i.e. only running the typechecker), and I've successfully run hh_client on sshfs-mounted (osxfuse) drives before. Is there another problem I'm not aware of that makes this a bad idea? If not, how might I enable hh_server --check to run on my VBox NFS shared folder?
The main issue is the lack of inotify support for NFS, so hh_server may respond with stale data.
If you accept the risk, you can add enable_on_nfs = true to /etc/hh.conf, which will enable hh_server to check folders on NFS.