Do all the "rotating external hard drives" that store these images mount with the same drive letter? If so, and if all your client machines can mount the external shared drive where you store the images using exactly the same file path, then store by reference should work for you OK.
PS. I don't really see how "rotating hard drives" makes sense for a back up scheme but that's a different question.
Thank you, that answers my question. The external hard drives have different drive letters at the moment, so it wouldn't work in the current state.
I should have expanded on the phrase "rotating external hard drives." The databases are backed up on a couple of external drives, one that is stored offsite, and one that moves between the site and offsite (these are the two that "rotate").
I wondered, that means you aren't hosting the file from rotating drives--that's the part that didn't make sense. We backup to flashdrives here for off site storage and I assume you'll need to do the same for the referenced files. I would think you could set up a dedicated drive or partition to store the referenced drive, keep it in place and make regular back up copies to a second drive for removal/off site storage.
That way, your referenced files all stay in the same place and you can map/mount it on all your client workstations in order for your store by reference container fields to properly reference the files.