I've emailed my Banjercito contact to see if he would be okay with me going to the Mexican consulate in Costa Rica to have them somehow verify that I have the bike here--and that it was not sold in Mexico. I'll update everyone on how this strategy works out.
Given that I'm now going to be flying the bike home, it may be tough to go back through Mexico--unless I flew the bike from Argentina to Mexico City, which may be possible. We'll see.
Fingers and toes crossed.
Thanks for your help.