I'd say try Sun Country, but......
Perhaps you can find out who is servicing their HMMTF/Mark Travel contract to Costa Rica (if they still are doing it). Imagine it is ATA or Champion.
Dont know the latest with Continental, but a few years back, "officially" you had to be on the International Jumpseat list, but EWR/IAH were usually cool about letting you on, and letting the captain be the judge. Be ready to spend a little extra time in SJ, and worst case, you might have to buy a ticket home. I never had to, but was afraid it could happen.
good luck.