I'm looking to redeem an Air France reward ticket for the transatlantic legs of a trip I'm intending to take overland between Cancun and San Jose. Cancun is no problem, but it doesn't look like Air France has any flights or code-share partner flights in San Jose.

So, I'm looking for an airport somewhere in the region, which has cheap flights to SJO and is served by Air France, or a Sky Team partner.

Any suggestions?

DL flies ATL-SJO daily

af flies PTP to SJO daily i think
2 flights to PTP a day from ORY

Air France no longer serves SJO. Closest service on AF is CCS or backtrack to MEX.

You can take the KLM flight PTY-AMS -- it's an MD-11 and a nice flight. I believe it's still 3x weekly.

Also remember Copa (CM) is part of CO's OnePass, and therefore a SkyTeam partner...

I'd agree the most reasonable solution would be Copa via PTY.
According to the SJO airport wiki (http://en.wikipedia.org/wiki/Juan_Santamar%C3%ADa_International_Airport), TA have "scheduled charters" between CUN and SJO, so you could also explore that.
In addition to the alternatives suggested here, there seem to be flights from HAV (CU and TA), BOG (AV and TA, with P5 due to start in Dec) and SDQ (TA), all of which have TATL Skyteam flights.
Of these, BOG probably is your best bet.