How do you get permission to visit Saihoji? Can you just show up?
You need to book in advance via an written application.
I have only tried to do if from within Japan, but if you google "Saihoji Moss garden Reservation help" which offers help to do it from abroad. Hope this helps. It may seem a bit troublesome, but it is really worth the effort!
Get quick answers from Saihoji Temple staff and past visitors.