Info from DCaDftMF3e p.54: Card slot identification signals
Identification signals 3 through 0 (/ID3-/ID0) are binary encoded to specify the physical location of each slot. The highest numbered slot ($F) has the four signals wired low. The lowest numbered slot ($0) has all ID signals high.
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. . . each of the 16 possible NuBus card slots has an address . . .
This would confirm my guess that "NuBus Slot $D encoding could be a single bit Delta value for /ID0 on pin C26."
Looks like I just need to make sure the top slot of the Gemini Card IDs as the IIsi's native $9 (instead of the Duo's $C) and then just pull /ID1 low on pin A26 to morph the lower slot into the IIsi's $B slot assignment. Same for the /30!
Does this look right? I always wondered why the Mac's IDs jumped from $0 to $9, it never made sense before.
edit: just thought I may have it backwards. Would the normal states for signals usually be high and pulled low or where "wired low" is pulled pulled high for encoding?