I figured I would take one for the team and just buy that Route 66 harness so I could get a closer look at the maple adapter... I got lucky a few days later the Route 66 microphone I'd been searching for magically surfaced in the wild (on eBay also).
Here are the close up pics of the microphone adapter / harness bits. It is indeed using an off the shelf DreamCast adapter FWIW.
The next step is identifying what exactly it is plugging into.
Having looked at a few Naomi1 manuals, I suspect the "840-0023D-02" is just a Naomi2 with a specific region setting. I have an "840-0091D-10" now. (I've since confirmed that my current board is an "OTHERS" region Initial D Naomi per this manual
http://store.playitamusements.com/Manuals/Initial D 3.pdf)
Likewise I never noticed that CN7 maple bus has 11 pins, where as CN6 only has 10. I am guessing this connector plugs into CN6 since the diagram shows a 10 pin connector.
Update:
I just went a head and YOLO'd it up, I removed the Microphone Enable and ground from the JAMMA edge (guess this was from a DLX cab vs a Naomi Upright?) and pulled the cable out of the bundle.
I'll try to connect it to my New Net City control panel pinouts so it can make its way to the JVS IO board.
I'm guessing the AMP connectors use a familiar pinout even though this is a "driving" cab. Pin 50 should go to GND, and pin 95 to "MIC SW". Once I get it connected and the Mic shows up, I'll check back in.