It seems like a valid question... it may be that in order to get miles (in whatever scheme) on the underlying ticket it is necessary to use the AA code.

(Example: if the underlying ticket is booked in, say, N class using the AA code then it earns 100% mileage credit on AAdvantage, but booked in N on CX it wouldn't earn miles in AAdvantage or AsiaMiles.)

I don't know the answer to the question though, sorry. However, I suspect that CX's answer might be that if the underlying fare isn't in itself eligible for AsiaMiles then it is not possible to upgrade. That is certainly the case for CX-coded flights.