It looks to me that DimScreen written by Skrommel does not lower down the screen Brightness, but does some other magic instead.
So I suggest:
In Windows 7, you can go to "Adjust screen brightness" and modify the screen brightness without a need of fn keys.
There exists a Backlight Control Interface:
There exists VB.NET example:
Would it be possible to implement it into AHK? If not then I will try to do my best to code it using C#, F# or anything else, so there would be an option to adjust a real LCD brightness But I must warn you that I have a zero experience with Windows API!
PS: One additional idea: zero brightness = turned off LCD screen