In reguards to attacking a model which is a character on a monstrous mount where you must target specifically the rider or the monster the bane head will work only for attacks to that target. So, for example, if you are able to do three wounds onto a target, two of which kill the dragon rider then the remaining wound would NOT carry over onto the dragon as it was not your origonal target.
However, if you target the mount Bane head will still work since it's considered the same model.