I am sorry to report that my quest to verify the entry conditions for the DC lounges in MEX was a dismal failure.
My Mexicana flights arrived and departed from T1 ( I was flying the $ 287 MX deal to Buenos Aires ), so I wasn't able to access the airside lounges in Terminal 2.
The Salon Premier lounge in Terminal 1 is currently closed for renovations. The sign on the door said it would reopen in December. There was nothing to indicate entry conditions with the DC card.
While wandering around T1 we did stumble across a couple of small AMEX lounges ( Platinum and Gold ). And made good use of the Gold lounge. They are located behind the Duty Free shop across from Gate 19. There is more information about these lounges in the AMEX forum if anyone is interested.