There is no need to call BA or to mention the EU Regulation to an agent who won't know what the passenger is about. BA has a very generous re-booking policy for cancellations. This far out it is the responsibility of the travel agent to rebook the passenger, and since AA is the travel agent all communication should be with AA. BA indeed does not require award availability for re-booking in the event of a cancellation or significant schedule change, which is clearly spelled out in the Customer Guidelines for travel agents. I would call AA again and ask them to contact BA's trade desk or their OW liaison if they're not sure that commercial booking classes can be used to rebook canceled award seats. It should not be an issue to get rebooked. Given the number of seats AA sells on BA, including redemptions, it should be a rare occasion when the agent is not familiar with the policy.