By way of data point: I was able to book a 50% off second kids room with an award stay for 5 nights at the Tokyo Shinjuku Hyatt Regency (I believe second room was $100). They also offered to upgrade us for a small fee so that we could add a rollaway to one of the rooms (family of 5).
Tried doing the same at the HK Hyatt Regency and was told rate isn't offered for our dates.