How do I use both a color's alpha and the texture's alpha in a shader pass?

I have a custom skybox shader (based off the wiki one) that blends three groups of textures - SkyBox 1, SkyBox 2, and Horizon Fog.

Here it is: http://pastebin.com/nN5HpsTb

Here's what it can do: http://picasaweb.google.com/aubreyfalconer/ScreenCaptures#5550634538857065698

It all works great, except that I want to add one more feature: the ability to control the horizon fog group's overall opacity with the alpha component of the _FogTint texture. This will allow me to fade the fog in and out as players enter a world.

Thanks!

-Aubrey