I don't know. AMEX should have the answer for you.. or at least CX would. I don't understand this. You booked the reservation while waiting for your Platinum card to arrive? You COULD have booked it without having your Plat car on hand you know... AMEX travel would have just booked it for you first. They would after all have access to your cc number first and that would indicate that you are a Plat member. On top of which, it doesn't matter if you're a no body when you booked the res, as long as you're holding a Plat card when you issue the ticket is when it matters.

Get Amex Travel on the phone, and ask them to speak to their CX sales representative to get that one seat for your confirmed.