It is possible to use the ZIP drive even without a proprietary driver installed (or else the Mac could not mount the drive from the SCSI driver stored in ROM). What should work anyway, is to use Harddisk Toolkit (HDT) to initialise the ZIP disk. Afterwards install a system onto the disk or just copy and bless a system folder onto the disk. You can choose the disk as a startup volume in the control panel. In case the disk is inserted in the ZIP drive on startup, the Mac should use it. If it is not present at startup, the Mac automatically uses the next valid boot drive it finds on the SCSI bus.

To modify the startup disk temporarily, use the appropriate key combo, as given in the List of startup keys.

Using HDT you could even choose a specific boot partition from your boot disk.