And this was it. In a somewhat sacriligeous move I hacked up a Hori GC pad, not knowing that years later they'd be somewhat rare.
...Actually I probably wouldn't have cared. =)
This image was for my own benefit: where to cut, without wiping out any components I needed, and what trace on the PCB was for what button. It was easier to do it this way than try to figure it out after cutting the PCB... In hindsight I could just as easily have made notes on a piece of paper...
I was quite lucky that every necessary component of this pad was in a small cluster. Once cut, I had only to run a handful of wires from this central area to the DB15 connector... Presto! Instant NeoGeo -> GameCube adaptor.
The final touch was, of course, to shove the entire contents into a gutted Dreamcast 3rd-party memory card... It fit remarkably well.