My understanding is that AA wants at least one transoceanic trip on AA metal, not just an AA codeshare.

As for using an AA code on a flight (JNB-LHR) when you're not connecting directly to an AA flight, I don't think that's allowed; you'd need to use the BA code. Next week I'm going CPT-xLHR-HEL and using the AY code on the CPT-LHR segment as I'm going on to HEL on Finnair. I don't think AA CPT (Dublin) would have approved the route had I asked for an AA code on the CPT-LHR flight, since I'm not taking AA across the Atlantic until a few days later. Codeshares seem to vanish if there's a break in continuity.