Transparency and Reflections
I saw a question on the Area that prompted me to post this article, as it reminds me of some questions Wall Worm users have asked over the years. The question is how to create a material that has no reflections in the transparent parts of a material.
As you can see in the screen shot above, it is something you can accomplish. The example file above contains that scene. By opening the material editor, you can see how this is accomplished.
Setting this up to export into Source will come later, as for Source you need to render all the maps into bitmaps. For Source, the best way to accomplish this is to render a diffuse texture with the transparency and specular level rendered into the diffuse alpha channel. This way you can just use the VMT command to make it work: $basealphaenvmapmask 1.