OK, I misunderstood the issue initially. Hope the below explanation will solve it.
The texture used by an alternative material has to be tiled (repeated) somewhere in the model. In other words, there has to be some surface in the original model where the tiling of the alternative texture can be seen.
A good practice is to organize the alternative materials in material pools somewhere outside the building. A material pool can be a set of squares: one square for one alternative material, and the texture on each square is scaled so that it is tiled.
You can find more information on the issue in this post: Material picker problem - #5 by wojtek. Also, this fragment of the extension-related video tutorial shows how to create material pools in SketchUp: INTERACTIVE Walkthrough on the WEB | Shapespark for Sketchup - YouTube