My experience
I booked BKK to DAY in Premium Economy at AA.com. Using the JAL PNR provided by AA I was unable to access my flight, so I could only assign seats on AA metal.
I called AA’a Thai customer service number to sort out a schedule change and when I received my new itinerary, the agent I spoke to on the phone assigned me seats (without me asking) in line with the seats I choose on AA metal.