Most of the links are to the OS X only version, or links to the old Iomega site which no longer works.
That said, I've resolved my problem. I had an epiphany this morning, remembering that I have Intech's Hard Disk Speedtools suite installed on this Pismo, and included removable cartridge support. I wondered if that is getting in the way of the Iomega utility to control the Zip drive. So I went into Extensions Manager, disabled the HDST Extension for removable cartridges and rebooted. Now working fine with the 3.5 version of Iomegaware.
Mean culpable.
Thanks for the assistance -- reading the exchange helped me think all this through. As much as I can be nostalgic for the look and feel of the Mac pre-OS X, troubleshooting extension conflicts isn't among the sources of nostalgia.