Just found this post. Had same problem and had trouble finding a solution. Here is my documentation on it:
IN SKETCHUP -- Use a script to Paint faces of model with materials based on their layer.
If you need to work with materials in SketchUp, then do the following to materialize faces based on layer designation: Download -- layers_materials.rb -- (Script By: Didier Bur) to paint faces with layer color:Layers Materials: http://rhin.crai.archi.fr/rld/plugin_details.php?id=219 IMPORTANT! After loading script, you must first run “CREATE MATERIALS FROM LAYERS” Then, run “PAINT FACES FROM LAYERS”
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http://sketchucation.com/forums/viewtopic.php?f=18%26amp;t=65107