We recommend using a masonry lintel (bond beam) in the CMU wall and a loose lintel in the veneer.  This will allow wood blocking to be located at the window head and the water/air barrier to be wrapped over the blocking and into the opening. Doing so significantly reduces thermal bridging at the window head. Overall, this method provides a more durable detail than introducing dissimilar materials, which can generate long lead times, unnecessary cost, and differential movement.