Originally Posted by JonathanIT

This entire situation is a very bad idea. BWB/FFN nights are not transferable. Diamond status benefits are for the named member only. Trying to use BWB nights in someone else's name and reap their Diamond benefits is not very wise; if the Diamond HGP member you did this "trade" with suggested it was not a problem, I would contact them and rescind this trade asap.

You will be putting yourself (and your GF) in a pretty compromising position in Tokyo. Unless you have other arrangements for accommodations or are prepared to pay the rack rate at the Park Hyatt, I certainly wouldn't attempt it.