I really think any texture should be possible as a default material texture... now it's "nil".
Let me keep talking in terms of GUI Dan, as I cannot go any deeper, though you help us understand underlying logics.
If there is no material, then there could be a new "neutral material" which could be overriden by whatever material is applied to a component instance.
Now, for what I imagine, colored materials (materials without textures only colors), also have no UV so the new "neutral material" shouldn't follow a colored material logic.
This "neutral material" should have UV so I imagine it requires a texture (maybe not but it helps us position it in the model).
If the texture is a 2x2px image or whatever else, for me it's nice, as long as it keeps it's current behaviour and doesn't impose itself as a checkerbox, upon our drawings, specially on the ones of people that don't know what's happening.
However, the best solution for me, would be to have this texture user defined on styles manager's face settings.
Following sketchup's current logic, we should have a texture for backface and a texture for frontface.
Of course if this texture is a plain color then it mimics current behaviour, apparently, but still could be positioned.
So, one could choose a color for backface and a color to front face and in fact we would be coloring a texture.
In terms of modelling/texturing in sketchup, texture position would also work, for the default material as it now works for a texture applied to a face.