I'm going to guess (hope?) that those flying on Basic Economy fares will be more likely to end up in these seats, assuming nobody else voluntarily chooses them.

Of course in IRROPS, yeah, EXPs could end up there; maybe they could withhold BE assignments until boarding starts. If they charged for paper BPs ("forcing" people to use the app), that would allow it to be relatively seamless. Probably too much of an IT headache for AA though.