To those who posted above, thank you. As it happens, I knew about the BA - GRU-EZE flight, but I didn't want to go to either GRU or EZE and neither one is reachable via a direct OW flight from GIG, my starting point. The only solution would have been to buy a GIG-GRU ticket, but that would require staying overnight in São Paulo because the BA flight leaves GRU at 7am. As for AA partner awards, the old awards on individual carriers don't exist anymore. There are only all-partner awards and these do not allow any stop-overs, or so AA phone representatives have told me repeatedly.