I have to agree with Buffy on this one, I love the idea of it and everything.. I think it would make things much easier, and maybe it'd work well if we could have that but also still be able to do presets via GIMP, Sai, Photoshop etc.
I personally wouldn't like doing it on the game as the options are extremely limited, unless "brushes" and all that were to be implemented as well.. I don't see presets coming out very.. good. I'm sure doing this would be a whole other level of coding compared to the game I feel like as well.
Other than that, the idea of this would be real cool!