Upgrading really isn't an issue since the flights on a MHD directly into/out of MEX won't be on AC anyways, so can't be upgraded with certificates or stickers. The AC sectors will all be domestic and/or transborder, so a V fare will work. It is only the non-stop YYZ-MEX service where your question comes into play.

According to the rules on the Cdn+ sticker folder, Mexico is considered as NAmerica for purposes of upgrading on those fares: but beware of the V-class problem as I have posted it elsewhere. As to AC certificates, that is an interesting question based on BB's response. You could always argue that the NAmerican Free Trade Agreement includes Canada, the U.S. and Mexico, so it would be hard for AC to reject upgrarding with NAmerican certificates.