Mexico City / MEX specifically and solely as a Mexico / North American destination is treated no differently than if traveling US to LHR, EZE, etc. by AA and by oneworld.

“oneworld lounge access for frequent flyers by tier status

American Airlines AAdvantage® members, regardless of their tier status or class of travel, are not eligible for lounge access when travelling solely on North American flights within or between the U.S., Canada, Mexico (except Mexico City), the Bahamas, Bermuda and the Caribbean. “ U]Link[/U]