Same issue, no resolution. Booked an AA award flight using BA Avios. It did assign me seats, but If I try to change seats on BA.com, it simply redirects me to AA.com, where the entire seat map is grayed out and I cannot change seating. If I log in to AA.com using AA PNR, or through my account on AA.com (Had Aadvatage # added to rez, so I can see it my "my trips"), the entire seat map is still grayed out. Called AA, no help. Messaged AA on Twitter, they told me I had to go through BA. Haven't yet called BA, but if their website redirects me to AA, I assume their reps will tell me to contact AA. One person on the rez is AA Gold, so we re hoping to choose some preferred (not MCE) seats, close to the front.

It's almost like they do treat it one step better than Basic Economy, as it did assign seats, but I cannot change them at all.

EDIT: Just after I posted this, I received a response to my response to AA Twitter. I had replied them back saying that BA was redirecting me to AA and that surely someone at AA knew how to fix this. Their response was, "Sorry for any inconvenience this has caused you, we were able to switch you to the seats you requested.

This falls under to age-old adage that if you don't like the first answer, call (or tweet) again.