On the day of your return flight to GRU you will commence the traveling from LAX or MIA? If the latter, I believe you will be able to obtain to check-in and get a BP for the MIA-GRU flight. If you are supposed to travel from LAX you may face issues when you try to commence traveling after no showing for the first AA leg(s).
But give it a shot and see what happens at check-in. Do remember to report back. Thx