Seems to me that a ghoul's hunger isn't necessarily like the RCI's. An RCI is tempted to drink blood, but doesn't need it to survive, however ghouls eat meat to live. So it's kinda like asking if the tattoos of saint giles would work on a mortal's hunger for food.
You could very easily do a PC (or "nice") ghoul by simply giving him a reason to play nice with others (and an easy source of fresh meat).