I went back to that image yesterday so I could do some more experimentation. I saved a couple more variations; the first was using smear-based brushes (colour smudge engine), which work quite well with the lighting effect. I don’t just mean the look of it - it also has an effect on the feel; something about the visual feedback as you paint that’s hard to explain.
The second was using a clone brush to see if I could get more of a natural dirty-brush effect; following the discussion in another thread. I’ve actually been very pleasantly surprised so far - especially after doing some proper sketching with it last night. 
