Do you mean render out something like an alpha mask for your uvs? If so, the quickest way I know is to use the "Batch Bake" tool. you can find it under Lighting/Shading->Batch Bake. Go into the options for that tool and set your options to:
Objects to bake: Selected
Bake optimization: Single Object
Bake Shadows: unchecked
Color Mode: Light and Color
Prefix: (whatever you want the file to be called)
X&Y resolution: self explanatory
Bake to one map: checked
Bake Alpha: checked
Alpha mode: Surface transparency
With your object selected, hit the Convert button and let it go to town. It'll bake your map out to [Your Project Folder]/renderData/mentalray/lightmap
The alpha channel of that file should have what your are looking for.
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Objects to bake: Selected
Bake optimization: Single Object
Bake Shadows: unchecked
Color Mode: Light and Color
Prefix: (whatever you want the file to be called)
X&Y resolution: self explanatory
Bake to one map: checked
Bake Alpha: checked
Alpha mode: Surface transparency
With your object selected, hit the Convert button and let it go to town. It'll bake your map out to [Your Project Folder]/renderData/mentalray/lightmap
The alpha channel of that file should have what your are looking for.