I have upcoming AY flights on BA codes. The chat agent responded that it probably won't be possible to assign seats but he'll try. Ended up getting my seats, but made it seem like he'd had experiences with both outcomes.
Suppose the systems may work differently depending on cabin, FF status, the other airline involved, phase of the moon, and whatnot. I even had an AY coded AY operated flight bought as part of a CX itenerary that was not availlable for seat selection on the AY site (didn't matter as I had been able to select seats at the CX site when booking, but seemed weird).