Ross
All I'm doing is tracing a photo in xara, copying the result and removing the fill from one and the outline from another and superimposing the two. Using your method I inserted a copy of the fill only version with a fractal stained glass transparency, (instead of clouds).
I used a plugin filter to apply a grain to the base image and made a bitmap copy of the outline to make it more managable (4000+ shapes slowed things down a little), this also softened the lines a little and made it easier to adjust the transparency of the lines to make them a little less dark.
A base image of paper texture was inserted last.
Its not anything like as watercolour (ish) as yours but added the interest to anotherwise boring image and gave me the inspiration for the canal building photographs I'm fiddling with.