I'm trying to get the lines to emphasize the light direction, but can't seem to control the Alpha in anyway to do that... or fake it.
You can fake it with the Distance from Object modifier.
The distance from specified object will then control the transparency.
1) In your Freestyle Line Style go to the Alpha tab and add the Distance from Object modifier.
2) Set the Blend mode to Subtract
3) Set your Target object. (from which the distance will be measured)
4) Set Min/Max Range for best result. (or use the Fill range by selection button)