Nice feature request. As a workaround you can also select box blur in the shade parameters and crank the amount value up to around 100 and make sure the center of the shade layer is equal to the center of the object. you should get a black line then. Although not totally perfect around the corners, but that should not be visible while animating.
-The Paramethers will be: Amounth = 100
Blend Method = Behind (OBIUSLY) Origin = 0,0 Size = 10 (size that you want can change) Type = Disc Blur (this one was the better define for the Stroke, the other blur methods left some noise blur arround the pic)