In computerese, a "Dongle" is a connector that has multiple cables coming off of it to connect multiple items. This "Jongle" I'm guessing, allows you to hook up multiple input devices to your Jamma based cab. I see a "video in", optical inputs, and not sure about the others. I don't see any VGA to CGA conversion, so I guess your source video needs to be compatible with the display in your jamma cab. I'd start testing with video - take the "RGBS-GND" outputs off of a game board, and hook them up to this, then plug the board into into your jamma harness in a cab to see if the video shows up. Next, trace out the edge connector back to the pins to see where your buttons and joysticks should plug in. Then, build yourself a fancy multi-input device that puts the cables from buttons, joysticks, maybe a steering wheel, video, etc into a parallel port. Then, plug the parallel port into your device to see if they work. The boards looks like it just makes hooking up an array of input devices and mapping them to a JAMMA pinout as easy as plugging in a single parallel cable.
Could be useful -- I frequently use boat loads of alligator clips to make my connections -- This would be so much easier!
I know you've got the wire to build it -- if not, I can sell you some more 22AWG - 38 different colors to make this. I think a parallel port is 25 pins, centronics is like 36...