hello, I actually did the projection painting script a month ago or so, look at it here :
I used the same idea as you
the script makes everything a little easier:
it makes an uvlayer and a new image. after painting, it bakes the new layer back into the original image.
The only problem is that you cannot really see the first layer if you don’t have a good glsl support.
I quite often use blenders texturing tools, and I’d really like if it would be looked at more in depth…