The AA site wouldn't have allowed booking of that itinerary - plus with AA it is unlikely that it would have been a valid routing
If you want to look at the reservations on other systems ( e,g, the cathay site to do seat assignments for Cathay ) you need the Amadeus reference. If you look at the reservation on the US site, it may show the QF/CX reference ( they will be the same ) ; otherwise call US, QF or CX and can get the reference and then do seat assignments on the QF and CX sites