So I went ahead and built a prototype harness. I grabbed one of these:
and 4x of these:
I simply pushed the .187 connectors for the 2-wire harness into the rubber shrouds on the matching connectors on the Crown Conversion Harness, matching black to earth and red to live. I connected the other end of the Crown Conversion Harness into one of these:
Which was in turn connected to the Sanwa JLF stick.
The 2pin JST connectors were then plugged into the board. With the Sanwa JLF mounted in the case so the 5-pin connector is pointing away from the buttons, and using the coloured wires on the Crown Conversion Harness as a guide, mine maps as follows:
Red -> Down
Yellow -> Left
Orange -> Up
Green -> Right
This is working perfectly. Minimal hassle, zero soldering. Yes there is some excess cable, and yes now I know this works I will be creating a smaller, soldered harness, but this would be great for someone who wants to connect a Sanwa to the NES30 without doing anything complicated.
If I was going to leave this harness in place, I would probably tape up any exposed parts of the .187 connectors just to ensure I avoid any shorts, but the majority of the connectors are well within the rubber sheaths so this shouldn’t be an issue.