Just a side note to all those attempting to directly access the back buffer in D3D. Make sure you have your swap effect set to D3DSWAPPEFFECT_COPY or you will get a very annoying flicker.
Also make sure that you specify your back buffer is a lockable one in your D3DPRESENT_PARAMETERS structure.
Since you are only locking the buffer once per frame, you shouldn't notice too much of a hit. You should not need to unlock the buffer until you are done with the entire scene.