@headwax – that image is awesome! I'm not sure that you want much of a bleed on the shadow. If anything, an irregular increase in density near the edges of the shadow would look like a traditional wet-on-dry painting technique. It would be visually cohesive with the rest of the look that easy and free, but has precise detail too. 

When I got into this business, I used to work in airbrush and watercolor...I really miss the watercolor.