There is no phone-reservation system for XE jumpseats. You just show up and fill out a form...hopefully the station has a jumpseat form. If not maybe you can ask if the crew is carrying some (I keep a few extras in my flight kit for such scnearios).
You need to insure that they put you on the passenger manifest and that you clear customs as a passenger and not a crewmember. I'm sure you already know this but only a cabin seat would be available to you. Also, I don't think you're exempt from the mexico departure tax, IIRC it's about 30 bucks, cash only.
Anyway, good luck, hopefully CAL ops in SJD knows how to handle this. If you need it I can PM you our jumpseat coordinators contact info.