The family and I flew IB MAD-ORD on an AA ticket a couple of years ago.
AA couldn't do anything to request seats, and IB could see the booking, but couldn't do anything with it. Despite numerous calls to AA and IB agents, despite being OW Emerald I just gave up. Ended up selecting the seats when I checked in.
If it's stiil the same, my advice is don't waste your time. Computer says no.