Awesome news for DCA regulars.
I suspect they might take the space from one of the Admirals Clubs when AA move into their new digs in the renovation.
I don't see a problem with it not being accessible from A, that terminal caters primarily to WN and F9, which (a) aren't a partner of Amex for cobrands and (b) typically don't carry the type of consumer that holds a Platinum.