If: 1) you don't want to or can't go the representation route, 2) you don't mind a performance penalty which could be almost unnoticeable, and 3) you don't mind that your line will block out any line under it, you might make your outline a combination of two lines, one a solid line with the color you want to see and the top one a cartographic line of white color with a template pattern that when combined with the solid produces the pattern you want.