I don't think that's something that Carrara can do out of the box; however there are a few plugins that will (more or less) help you get the effect.

"Shaders Plus" from Digital Carvers Guild allows you to control how each light in your scene affects each shading domain of an object. For example, you can add a light that only casts specular light on a characters eye surface while completely ignoring the rest of the character.

"Carrara ERC" available at DAZ allows you to use attributes of nearly anything in your scene to control attributes of pretty much any other thing in your scene; if that makes any sense...

I'm almost positive that there's a plugin out there that does exactly what you're asking; maybe "Shader Ops" from DCG or one of Inagoni's add-ons.

Mark will probably know!