Inside the drive is a raw Sony DSDD floppy drive, the same as found in period Macs. It is connected to a board that holds the DB-19 pass-through to connect another drive and the cable that goes to the computer. This board also seems to handle the eject button in addition to interpreting the "3.5DISK" signal on pin 4 and the Drive Select Signals.

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I think I get it. This board has a cable to the IIgs, an IDC-20 connector for the floppy drive, and a DB-19 pass-through so you can optionally daisy-chain a second floppy drive, right? So Floppy Emu didn't work when connected to that DB-19 pass-through, but did work when connected to the IDC-20, replacing the real drive mechanism. Makes sense.

I wonder if the eject button will "eject" the mounted disk image if pressed.

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Given what you've told me, I'd guess it does. I can also add an eject feature to the firmware, that's pretty easy. It might be a little confusing for Mac users to see an "eject" option, though.

So yeah, I'm very interested in what modifications to Floppy Emu would be necessary to make it work on the IIgs without requiring that daisy-chain board. Ideally while still maintaining interoperability with other real 3.5 drives in the chain. Does anyone have a high-res photo of the daisy chain board? Does it have any ICs on it, or is it just a bunch of passive components?