I also reached out to my hotel concierge at the Park Hyatt and they told me they were told by the museum they can't book on the behalf of foreigners. They said they'd try one more time to talk with them and see what they can do but the expectation is that the museum will just give them a similar answer again. Quite the bummer and I wonder why the museum would be against a local hotel buying tickets for their guests. We may have to look into seeing if we can find any connections in Japan that can help us.