So, this is very good news that the Amex Plat should get entry. I have been in touch via email with both Priority Pass and the new T5 Plaza Premium Lounge staff as well as with American Express travel benefits and I am fairly confident that I can confirm there is access to the T5D lounge using the American Express Platinum card without having visited there. (I expect to be there Sunday afternoon.)

Here is the answer I got from the lounge staff directly: "Regarding your enquire, we do accept American Express Platinum card. However, what concerns the charges, we strongly suggest you to refer
to the terms and conditions of your bank account, as unfortunately we do not have that kind of information."

That was fairly unclear as to whether the card provides "free" access or if they accept the card for payment of entry fees. So I called American Express to see whether they had any information. I was told with good certainty by the agent I spoke with, who consulted with the travel team, that the Platinum Card would provide access to the the Lounge as a benefit.

I will note that on the Plaza Premium Lounge promotions web page there is this promotion listed...

"Enjoy unlimited Plaza Premium Lounge access with the beloved ones with American Express Elite Credit Card"

I asked the American Express agent whether this referred to a specific American Express "elite" card that I had never heard of, and she said "no, we don't offer an 'elite' card, we offer Platinum and Centurion cards which some may consider elite cards."

So, all in all, I am feeling fairly confident that I will be able to spend a couple of hours at the Plaza Premium Lounge in T5 on the departures side.

I hope others find my obsessive research helpful.