Bobby--when I first looked at this question about 6 months ago, I think TACA ran a flight out of the El Salvador to Cancun at that time. I distinctly recall both checking the flights on the TACA web site and checking the GlobalPass award website, and it showing the necessary mileage. Now, however, TACA shows no flights from either NY or Miami to Cancun. Checked next winter too, in case it was just seasonal. Looks like they do not offer any service to Cancun any more.
Looks like this is no longer a possible option for awards.
Djlawman