As a PLT (or any AA elite from my experience) you can use the Clase Ejecutiva counter and board with Zona-1.

Upgrade wise I have scored 3/3 on paid counter upgrades JFK-MEX and 2/2 on MEX-JFK. Recently I did a trip JFK-MEX on MX (sold as AA), MEX-DFW-EWR with AA. Paid $150 for upgrade from JFK-MEX and it was well worth it. On return when I checked in with AA @ MEX it took 45 minutes for the agent to figure out what MX did with the upgrade. Apparently, the MX agent who checked me in @ JFK used all the coupons. So, just be careful. If you're doing roundtrip with MX then you'll be fine (I've done this, and upgraded both ways). But if you combine MX and AA you might have problems.

Also, if you ticket the flight as an AA flight and upgrade the miles will not post to your account as the AA codeshare flight. They'll post as MX### and will not give you EQMS. You have to call AA and explain what you did and they'll fix it for you.

Let me know if you have other questions.