Plan A: I would run the wires from the switch panel forward in the wire chase that is at the upper corner of the cabin. At the mast, turn and run to the port side, then continue to the area over the head. There are removable panels along the way to pull the wires in sections. I have run a TV cable exactly like I described, stopping at the mast. I don't think it would be difficult to continue to the forward head.
Plan B: Or it may be possible to run a cable along the waste hose from the forward head, but I have never done that.
Alternatively, you could just string the wires directly from the switch panel to the door to the vee berth and then continue into the head. Much faster job, uses less wire, and this method would provide a "clothes line" to hang and dry bathing suits. Win-Win?

Well, maybe not..... Better stick with Plan A.