I have a complex ticket for the autumn which features BA, AA and US flights. I noticed this morning, presumably as part of the AA/US reservations merger, that some of the US flights are now editable via My Trips/Check in on AA.com.
For example this segment from Phoenix to Honolulu now allows seat changes via aa.com. In addition, Row 1, typically held back by US is now available for selection.
This is clearly the old US-757 configuration to Honolulu:
If you have a bad seat on a former US flight it is probably worth checking as you might secure something better.
I noticed this working when I select my reservations via ba.com and then ask it to switch me to AA for seat selection.