Seat assignmens
 

I recently purchased a multicity ticket on Quantas for next June. The last leg of the trip is on a flight operated by AMerican Airlines flight QF3019 LAX to MIA. I tried to book the seat assignments but the system did not allow it. I called Quantas and they tell me that American needs to assign the seats. I called American and they tell me that Quantas needs to assign the seats. After 5 phones calls and several hours trying to sort this out, I am told that the "system" will not allow seat assignements until we get to the airport on next June.
I am getting no help from those idiots in either airline. Any suggestions?