How to backup GitLab omnibus when it's VM image is mounted on another VM?

I have a GitLab CE omnibus install on an Ubuntu 16.04 guest that’s hosted on FreeBSD/bhyve. A recent unexpected host crash has rendered that VM unbootable. Other similar VMs boot OK, and I can mount the GitLab/Ubuntu image on another VM and it seems to be fine.

Is there a way I can start GitLab when it’s image is not mounted as /? I’d like to backup and restore GitLab to a new VM, but GitLab (at least postgresql?) has to be running to do a backup.

As a workaround, I imported the old VM image on another host and was able to create a GitLab backup.