FYI, this is the number I was given/use and I am always connected to a "Marriott Luxury Associate" who can look up/book the club upgrade rate. It's not just central reservations. Did you speak with someone after dialing that number?

I totally agree that it's unfortunate we cannot even look up these rates online, that properties can be very selective in which rooms (if any) they open up for the club upgrades, that the upgrade rates often vary from the member flexible rate, AND that the Ambassador service cannot quote us these rates via phone or email. I've also had issues after booking, where my reservation reverted to one night (instead of four) two weeks before arrival - that was a mess!

That said, I have still found some great value with the certificates, most recently in October, when the club upgrade rate ended up somehow being lower than the regular rate. Quite a crapshoot, however. :/