Nice try, but I think they've got themselves covered. See the bit below, from the One-World page devoted specifically to lounge access, I added the bolding. Would anybody suggest that, while flying SFO-LAX on AA I would be given access to the BA operated Terraces lounge at SFO as a Plat? I didn't think so. But that's equivalent to what the OP suggests with the 'Qantas' lounge at LAX.
AAdvantage PlatinumŽ
Equivalent to oneworld Sapphire
Lounge access benefits: When travelling internationally* on any of the oneworld member airlines, regardless of the class of service, a Sapphire equivalent member and one guest will be welcome in any of the exclusive airline pre-flight Business Class and Frequent Flyer lounges.
* Solely North American itineraries within or between the U.S., Canada, Mexico (except Mexico City), the Bahamas, Bermuda and the Caribbean are not considered international destinations. This exception applies to AAdvantage members only and not to any other oneworld Frequent Flyer programme members.