I was able to book a room for three nights on points for Xmas (25-28 Dec); albeit only for a twin, not king, room. The cancellation policy is 14 days, so I still have a few days to decide whether to keep this room or not. I am already booked into the Centric Ginza on points for the same three nights.

The Park Premier Suite was also available on Points+Cash, for 45,000p/night+JPY 236,667/night. However, looking at the photos, this seems to be the "old" Park Deluxe Suite which was the basic suite for upgrades before. I hope the hotel has not reclassified these Standard Suites into "premium", and thereby excluding them from the upgrade pool.