I'd say, if you can get which pin is ground and which one is +5V (follow the traces), all the rest should be a button or a lever direction, it would just be a matter of shorting them with ground to get which is which...but I'm sure James would have think of that so, what am I missing here?
Regards.
You are making 3rd and 4th player harnesses? Interesting, I will contact you sometime for cps1 and cps2 3 and 4 player support to Neo geo controller.