Each system has a country lock out built into it, if the cartridge is made in japan for a jap Saturn then it wont be able to be used with a U.K or U.S.A saturn. All it can do is allow the use of Import CD's so using the jap PAR in a jap saturn and then puting in a U.S.A CD and then using the import feature of it will allow the use of that game.
To my knowledge these cards can work in any saturn, I have never seen anything to prove otherwise, especially since most PAR's are built in hongkong and I've seen people from america, england, and other places use them.
It's possible, but I don't see why anyone would do it since AFAIK the original AR carts made in England by Datel had multiregion booting from the beginning, so anyone reverse engineering/copying code from that would have to deliberately remove the signatures for other territories. It's probably a faulty assumption on the part of the seller.