Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Avios points can only be used to secure AA award seats that are available at the MileSAAver level.
AFAIK, every MileSAAver seat is available to partners like BA; I do not believe that AA holds back any MileSAAver seats from partners.
The ba.com website does not always do a good job of showing award availability on AA connecting flights; I would search for each individual AA segment on ba.com.