A 787-8 could fly DFW-SYD, with enough slack in the range to fly around the ETOPS-180 no fly zone. Since Qantas went with A380s and they're not getting them for a while, they can't fly that route with their fleet and thus shouldn't have any complaint about AA flying it. The only catch is that AA hasn't been buying aircraft, and upgrading their domestic fleet to something more fuel efficient than the Mad Dogs might be a bigger priority (although maybe not lately since fuel prices came down a bit).