Actually ozstamps got upgraded from LAX-SJD on MX and Gaucho100K said in this thread http://www.flyertalk.com/forum/Forum82/HTML/000226.html "I've just been told that Mexicana gives its Frequenta Gold members, and *A Gold members automatic upgrades to C for all domestic travel (subject to space available).

Gaucho was going to check this out and inform. Note that he said only "domestic" but ozstamps has received the upgrade for an "international" (maybe Mexicana considers US-Mexico travel domestic? or maybe he was just lucky). I certainly felt great getting upgraded when we did the trip together in October Ken and those were international flights too.