Not sure if it's the same on CX, but I had a similar situation with BA. The ex-SIN inventory for SIN-LHR was D0 for both BA 12 and BA16 but they were in fact wide open if viewing ex-UK inventory (EF and AA's systems are only able to view inventory from an exSIN perspective apparently). I confirmed myself onto a flight that was available (can't WL in D with BA), called up BA UK with the Amadeus record locator and got them to replace the confirmed flight with the one I actually wanted. Now whether the BA agent bent some rule in intervening in my as yet unticketed AA-organised itinerary is another story...