I gave birth naturally to a posterior baby. So, it can be done. (It was painful but it was my only natural birth, so I have nothing to compare it to.)
My obgyn did have to break my water, but I don't know if that had anything to do with the baby being posterior or not.
My doulas put heat packs on the small of my back, and my dh pushed on the small of my back, through every contraction. That really helped me get through the pain.