Looks like that option is only available for models. One other problem would be that my shapes are given to me dynamically by a 3rd party system (users can create their own shapes with fills and line styles, and that data gets passed to me), so I don't know ahead of time how many times to make it repeat for it to look right.
Does anyone know if making a repeating pattern on a canvas and then using that canvas as the fill image would work? I am trying to get that working (I can make an image repeat on a canvas exactly how I want it to), but I can't seem to make an image property take my canvas so I can't yet tell if that would work.