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Ok here's the test I did.  I used VSS to channel the Skin material to Custom 1.  This gave me a render that I could load into an alpha channel to mask it out.  I then used a Z Depth render with DOF Pro and changed the color of the image without changing the skin color.  What do you think?

Very slick!

There's another way you can single out the skin material as well, just as a FYI. You can go through all of the various skin materials in the character and assign them to the same Toon ID.  Now in my experimentation with Toon ID I found that the greater the distance between the numbers, the greater the distance there will be in the colors used to represent the different toon IDs.

So, let's say you have a character, and the clothing in the scene and you want to separate them from the background.

Assign all of the materials in the skin to Toon ID 100000

Assign all of the materials in the clothing to Toon ID 200000

When you choose your auxilary render info, make sure that Toon ID is selected. The layer will be called ID in Photoshop.