You can cut the trim above beadboard and into the beadboard to accept a mirror. If you would like to do it yourself, there is a great tool to do it, it is a multitool. There are several manufacturers of it, Fien was the original but Porter Cable and others have now made this type of tool. Your local hardware store should be able to help you with it.

I have attached a link to some pictures of a bathroom we completed that would give you an idea of how it would look. Though we did it in board and bat, it will give you an idea.

http://www.houzz.com/projects/951761/bellingham-wa-residence