In reality, this is all academic anyway. In the past when I've had to connect to an onward AA domestic flight coming from a CX inbound, the "rechecking in" of luggage isn't even really that. Basically, after Customs inspection, you just drop your bags to a conveyor belt. No weigh-in actually takes place. If there's any AA personnel at all, they're there to facilitate (they're not even really AA employees, just contractors) and are certainly in no position to take payments for excess weight. This has been my experience at LAX and SFO.