Well, I give up. There are many online guides for using a Zip 100 on a Plus, and I'm doing exactly what they say, but the drive is never detected by any SCSI utility, so the whole question of drivers is moot. I have to conclude that my 512K with the Brainstorm memory+ROM+SCSI upgrade is somehow not exactly equivalent to a real Plus, causing it to not work. Seems very fishy, though.

I have a real Mac Plus coming in the mail in a couple of days, so I guess I'll try it with that.