JPMays, yes it really can be as simple as creating a simple AGS and then adding your variable opacity layer below it.
However, with this method you loose the "translucent" effect by replacing it with an "opaque" (opacity) effect.
There is another way of doing this in order to achieve what (i think) you are looking for.
Here is how you would set up the example Half Life posted (which there's nothing wrong with doing):

it would render to this:

Another option you could try would be this:

which would render (faster) to this:

in this case, the glass material is setup like what is shown below and the translucent shadow plane is a single layer material with an opacity set to transmit the amount of light your AGS material would produce.

Another option would be to combine the material above with the material Half Life posted as shown below if you wanted to retain the light reflecting off of the windows:

it would render to this:

Furthermore, you could add the variable opacity solid layer into the hidden from GI glass material for additional tweaking:

setup like so:
