Good news everyone, I tried reserving a Kansai WIDE Area Pass on JR-WEST ONLINE TRAIN RESERVATION, designating that I would pay for the pass at the station ticket vending machine. When I went to the vending machine and scanned my US passport (not a temporary visitor), it allowed me to proceed, prompting me to insert cash to pay for the pass. On the other hand, scanning my Japanese passport resulted in a prompt saying that "this selection is not available" (or something to the like) and I wasn't able to insert cash to pay. So I believe that the machine checks only the nationality of the passport. I didn't buy the pass because the price of an exchange order is cheaper than buying it on JR-WEST ONLINE TRAIN RESERVATION but I'm optimistic that any non-Japanese passport holder can pick up the pass reserved on JR-WEST ONLINE TRAIN RESERVATION.

The only concern: when you pick up the pass at the vending machine, no staff checks the passport. But the JR West staff reserve the right to check the passport during the journey. Do you guys think that they might be a little upset if they realize that I'm not a temporary visitor? In that case I suppose I could just explain that I entered Japan using my Japanese passport so it's impossible for me to get a temporary visitor stamp, if I follow Japanese government guidelines that Japanese passport holders must enter Japan using their Japanese passport.

I will try to buy an E-TICKET exchange order that I can use at the vending machine and report back to all of you whether or not the ticket can actually be issued.