I would not go the permitting route. There was an obvious intent on providing the bath room when the structure was originally built. As mentioned by several, al of the pumbing rough in was already inspected.
With regard to getting sued, if your house burns down because of anything you did directly or indirectly with or without a contractor being involved, with or without a permit, you may or may not be drug into a suit regardless.
Just do it.
Also, permitting will cost you money from the cost of the permit, to the cost of getting the drawing done and it will liekly increase the taxes on you house... Goverment in action.