Really? I always thought it was a one-way thing (NA -> JP), It works on JP systems too?
Before I installed my RHEA, I used it on my modded (to boot JP region games) NA Saturn to play NA titles...again.
It works out nicely because NA games don't lean on the cart slot as much, so in the event you want to use a first-party RAM cart to play a fighter or something, the console boots JP by default.