Well, apparently this is quite a rare disk. I put out feelers a few other places and couldn't come up with an original.
However, I was able to jury-rig one by finding another Disk Tools disk image for another model, use Mini vMac to delete the System Enabler for the other machine, copy in the Duo's Enabler, and dd the result to a physical floppy. Worked like a charm, even if it's not the original bits.