Good news, I now have a JROK compatible version of the mini SLG!
For the wiring setup, I ignored the 2-pin header on the JROK board (V/H sync), and took the sync/video lines from the Jamma harness (Sync, Ground, R, G, B) and wired those up to a VGA connector, putting the Sync into the VGA H (pin 13).
Next I had to remove the Vsync signal (since it is not connected) from the output VGA connector on the mini SLG. Since Vsync was not involved in the circuit anymore, the switch, and several other parts could be dropped as well.
The result is a mini SLG unit that only has two resistors and the chip (see picture), and works great with the JROK when wired the way I described above (tested on 2 CRTs, and 2 LCDs).
Due to the reduced functionality (only works with the JROK, no off switch) I am going to sell the JROK Multi-Williams version of the mini SLG for a reduced price: $21