The LAN flight to/from IGR flight shows availability from Buenos Aires mid-May on britishairways.com, but don't forget that you need to specify the smaller AEP airport. BAEC charges 7.5k Avios while AA charges 6k AAdvantage miles each way (AA's "wholly within select South American Countries" awards, but like all LAN redemptions requires a telephone booking fee). From MDZ to AEP, BAEC charges only 4.5k Avios.

Over the years I've noted that SCL - MDZ award availability on britishairways.com comes and goes within hours. I've seen unlimited availability on three daily flights every day for a week, which then reverts to zero availability on any flight. It's as though one touch of a button causes all redemption availability for the entire route to be zeroed-out for long periods of time. I would advise leaving that particular segment until considerably later, especially since you have a very attractive alternative by bus.