I was given a couple of of these certs a couple of years ago, and applied it to my bill on a stay at the BSH, still paying the balance of my bill with my Amex. I would think that as long as you have a charge to your Amex, even if it's not the full amount, it would still count, but that's nothing more than a guess. In my case, I can't tell if it affected my PLAT5 qualification or not, since I had well over 5 other nights to maintain anyway. I think this year I may even have enough to hit Plat without the help from Amex.

I've always wondered if they ever check that you charged your stays to the Amex, or if they just verify with Amex that you hold a valid Platinum card and post 5 stay credits on your profile.