I think the behaviour is just another of Adobe's bizarre design decisions for the 3D features.
Create a temporary set of duplicates of the master texture file, so there is one file for each required material attribute. Load the files into the material then delete the duplicates. The material will contain independent textures for the various attributes.
Thanks! Your solution works great.
I just started working with Extruded 3d shapes, and 3d text, and, as it turns out, this is actually a dandy feature, since these 3d objects each contain multiple elements to apply textures to (ex. Front Inflation Material, Front Bevel Material, Extrusion Material, etc.) that would be pain to edit individually, rather than via a single Smart Layer, especially if you plan to use the same materials for each.
So, anyway, onwards to more adventures as I climb the Photoshop 3d learning curve. I've been a wee-bit frustrated that this learning curve seems to be more difficult because the Adobe Photoshop_CS6 reference PDF file seems to reference mostly CS5 and, more often than not, references CS5 interfaces. For instance, in the CS6 manual—the sections on 3d concepts and Tools, 3d Panel Settings, 3D rendering and saving—to name a few—all explicitly refer to Photoshop CS5 Extended! So, if anyone knows of a detailed source of CS6 information, I’d be grateful.