Have you tried calling them? I guess the only seats they'd allocate free of charge are in the lower categories, basically the middle seats.

There have been reports of AA denying AB members guaranteed oneworld benefits, such as extra baggage for Emeralds, so if AB had a competent IT department one could call this reciprocal benefits Then again, they don't have that. Which is probably the reason why you can't assign a seat for free in the first place.