Hi, interesting point. I would guess that the TA would refer you to the carrier whose ticket stock it is issued on. I have no idea but might guess at QF for your starting point. I would suggest that you try the issuing carrier first and then AA. Also, why wait until nothing can be done, ask now.

It would be useful to arm yourself with the following
1. did you leave a contact number in the booking that the carrier can reasonably be expected to contact you on to tell you of the rearrangements
2. where is the nearest available airport that AA or babyAA will serve that might be an alternative and how could you get there
3. does the lost segment matter to you in terms of miles and status points, you may be able to claim for some FFPs as this is invol reroute
4. ask if they will ground transport you (based on 2) or alternative carrier you, if booked well ahead then it might be cheaper to do it yourself and try and claim the money back from AA now.

Good luck