Mister Janchichan - I don't know if this helps, but locking the layer's transparency allows you to paint within the shapes you've laid down without losing the crisp edges - it's not a great example, but I quickly scrawled the black shape below, locked the layer transparency and then over painted with the magenta, without losing any of the shape definition...
i'm glad you're finding this technique useful. i've seen several awesome artworks popping here and there with it, and it's extremely fun and exciting seeing what peeps do with it.
speaking of Alchemy, i had a sudden urge to give it a try with a new fun semi-serious style i've been experimenting with.
free sketchworks done in Alchemy without constraints, then brought into photoshop cw5 for color passes. i did quite a few that i'm going to post here.
it's a bit of a side thing from custom shapes, but it's healthy experimenting in other directions, even though custom shapes are now fully belonging to my workflow at a point or another.