As long as you have a reciprocal agreement with the other carrier you can jumpseat (riding in the cabin or course). A word of advice though, once you get to the other country, espicially mexico, the gate agents will not know that you can jumpseat. If you talk with a captain he should be able to help you out, sometimes they have to call the company and get you listed as a non-rev. But someitmes the captain never comes into the gate area... As a backup I would buy some ID-90s or a Zed fare, you can always return it when you get back if you don't use it. Which airline are you using by the way?