Have you tried the MATERIALATTACH command? This allows you to assign materials to layers.... I just gave it a quick test, and the materials display on fittings as well as flexible duct.
In the Materials dialog, you can also attach a material directly to an object... as above, works with fittings and flex.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.