Updates:
SalvationData board was a dead end. Did the ATAPI mod to that SD board, and it responds identically to the way it does in the default IDE Mode (in other words, it doesn't do anything but light the Status and Power LEDs up).
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I found a gentleman online who manufactures and sells a board that was originally designed to emulate ATAPI Optical Drives for (I think?) defunct arcade machines, using USB Drives as a storage replacement. He was so interested in what we were doing, that after a couple of emails, he bought his own SP-808, and he is going to give adapting his existing board to our application a shot! He says he expects that his existing board will work, by way of a few small software enhancements to handle ZIP-specific commands.
He is aware of this thread, and of the concurrently-happening MidiManiac attempt to crowdfund an adapter. To me, this guy's work seems like it's by far the closest thing to a solution out there at the moment. I won't link to his page here -- but know that he is serious about pursuing this, and certainly I am sure he'll speak up if/when he wants to.

rs
Been bitten too many times to participate in crowd funding projects. If this thing is possible, it *should* be a relatively cheap microprocessor board. Someone just needs the skills to know how to make it.