In January 2013 we went London/Sao Paulo/Santiago on BA/LA in business. The connection at Sao Paulo was about 2 hours. As we got to the top of the jetway we found a little door and a staff member who checked us off a list and then let us straight into the departures area. We were right at the end of a pier so we wandered along until we found an information screen which told us the LAN flight went from the other pier (which is actually designated a separate terminal). We found our way into the other terminal (all signed), found the American lounge and settled. No announcements so we wandered to the gate, getting there a little too early but LA have a series of separate queues so we were on board easily.
We came back from Buenos Aires so I can't comment on the return journey.