Option a) SAN - EZE, stop-over in RIO/SAO
Option b) SAN - RIO, unsure whether any fare will allow you to go to EZE, included
If AA sells it and you want AAdvantage miles check a SAN-GRU and GRU-EZE with BA. BA flies this route on certain days of the week. Apparently AA has this fare over 1100 USD, (V-class). But then, this fare and the upper ones until 1300 USD won't allow a stop in Brazil - no good for you.
Seems Varig to be the most inexpensive (under 1000) and allowing a stop-over in Brazil at 60USD/each stop.