I got a call back from customer care. The agent insisted throughout that she couldn't add my Aadvantage number. She also wouldn't choose seats for me. She did say that the local QR office might add my ffn 2 days ahead of time. She said that all this was happening because of IT system updates to enhance the security of the system.

On top of all this, I got an advertisement email yesterday from QR. It said that complimentary seat choice was available for my flight. But when I went in to choose seats it still wanted me to pay 242 Saudi riyals (sometimes 343). Their system is a shambles.

Edit: I also pointed out that others, such as the post just above here and another one a few days ago, got their ffn added by the call center agent after pushing for it. This one wouldn't do it.