Do you know? Now it is possible to paint layered textures using glsl (apricot) build.
Every layer must be a texture and you must make a material that uses these textures with different blending modes.
Like Gimp or Photoshop layers? The kind you can visually see in a stack and rearrange by dragging?
Or the weird copy and paste style? Either way Awesome! Sounds nice and will be fun to learn how to use.
I assume what you mean is using Images with UV coord and different channels?
You can than also attach them to different types like color / normal / etc ?
Only what is missing is better paint tools in Blender 
what he means is that essentially you can paint like you can paint in photoshop and/or gimp with masked_layers.
use stencil texture in the middle of 2 textures as the layers mask.
video demo please!