This is to report a small bug on OMV 3.0. Basically, the label of the mergerfs file system should be showing up as STORAGE (the name I gave it in the unionfs plugin) rather than /media/<uuid>. The proper name shows up on the File Systems page, but not in any of the Device/Volume combo lists.
Not sure if related, but I'm running OMV in Proxmox with passthrough drives.