Iirc, the WZW Centurion lounge is operated by American Express, and is also free to AMEX Platinum - and probably AMEX Gold, like the MEX Centurion lounges - card holders. A couple of trips ago, the AA EZE-JFK flight departed Gate 3, one gate down from the Centurion lounge, much more convenient than the AC at the other end of the Duty Free gantlet. (See my update in the first post, as of 24 Mar 2010.)

I recollect walking in and spending a little time, but I don't recall the details (I do not think they had showers, pretty sure of that.) Sorry. I also think they would not let an Emerald in as a BA contract lounge, thoug it might be worth a try.