When passing nearby, I wondered what was going on with the “temporarily closed” notice, so I stopped by. I don't know exactly when it started... but apparently the toilet building had been sealed off and couldn't be used. That might be the real reason for the “temporary closure”... (It's unclear why it isn't marked permanently closed. I didn't get out of the car, so I don't know, but I couldn't spot any notices explaining the closure.) As for nearby toilets... there's a public toilet at the top of Sekihoku Pass (closed in winter), and I seem to remember some sort of toilet facilities if you drive toward Mikuni Pass... so perhaps you can manage if you get that far? ※Going to Sōunkyō Onsen is another option. I believe there should be convenience stores or some kind of toilet facilities there, so that would be more reliable.