I am also looking to find the answer to this question as I have an award stay coming up at the GH Tokyo and would love to request an suite upg if possible.
Since the Japanese hotels strictly follow T&C, I doubt that you will be able to upgrade an award stay with the confirmed upgrades.
If you have a paper diamond welcome kit upgrade voucher, you can try to use that on an award stay. Just make sure that you call the reservations center and have them note on your reservation that you are going to try to use a paper upgrade certificate.