Good news, bad news.
First the good news. The old GUI works just fine under Snow Leopard. I was able to set the device path. SheepSaver 2.3 and the latest build both mount the device. FYI, for those who do not wish to get their hands dirty with Terminal can unmount the volume in Disk Utility just as easily (as well as get the device info).
Now for the bad news. SheepShaver mounts the device but immediately gives me the old "This Mac Cannot Recognize This Disk ... Do You Want To Initialize" Then it gives me only one visible choice: "ProDOS OK" I can also select another position on the drop bar with nothing. I have not yet initialized the disk because it otherwise shows up on my Snow Leopard desktop as a perfectly readable HFS volume, and under Panther, a fully interactive HFS volume. Again I have not tested it on a vintage Mac yet. However, when I launch Disk First Aid under SheepShaver's 8.6, the ZIP drive shows up with a garbage name: "p!.. IȦ̇̃ ᴂ". Choosing "erase" also only produces ProDOS OK, or the blank option. Attempting to erase other volumes produces the choice of HFS standard. So I'm not sure where the problem lies.
EDIT: Some good news. I put in a ZIP disk which was formatted HFS on a vintage Mac. The ZIP disk that did not show up properly was formatted HFS under Panther.
Evidently the disk must be pre-formatted HFS standard in order for SheepShaver to mount it or erase it.