There's actually a tool made for rolling fenders.. Not sure who makes it though. It's basically a big roller mounted on an adjustable arm that fits on the brake hub.. just pop the wheel off, mount the tool like you would with a wheel, adjust the arm so the roller is right on the fender, and roll it around. There's one shop in the area I know of that has it, and according to the owner it's a damn expensive tool. The funny thing is, is does pretty much the same thing as rolling a baseball bat through the wheel well.... I've used this method on two cars and it works fine.
-R