If you have the electronic boarding pass (even with seperate tickets) for your onward flight, GRU int'l transfer security will let you pass through and head back upstairs to international concourse in T3 without clearing customs. there is no AA transfer deck, but AA will open up the gate about 1.5-2 hours before departure with staffing. You can just go to the gate a little early and show your passport to get the printed boarding pass. I have done this several times over the years with various airlines through GRU.