Is the wet wall (where the showerhead is) on an outside wall or does it back up to another room? If another room, maybe break into that wall from behind being careful not to beat on it too much. You may be able to then remove the pipe and resolve this. Have you tried removing the shower arm? This can be done without tearing up the tile. If you're lucky, the clog may be in there, but it's more likely in the elbow. But, with the arm out, you may be able to dislodge that. You'll need some pipe dope or tape to seal the joint.