You will have to resort to some compositing tricks to achieve this effect. Cycles can't really perform an "if [glossy] reflection" check like you describe since a glossy surface is always reflecting something. You want it to change the surface's properties based on what is being reflected. This is impossible due to the backwards ray-tracing nature of Cycles since changing the properties of the surface will affect what is being reflected, creating a circular dependency.
To create the effect you are looking for I created three render layers, the main layer (as you have now), a just glossy layer (to use the lamp's highlight as a mask), and a black diffuse layer (to be mixed in with the mask layer).
The only other change I had to make to your scene was swapping out the mesh light with a point lamp since using material overrides in the render layers also overrides the emission material. However, it basically looks the same and renders a bit faster and with less noise to boot.
First create two more materials. Create one material that is just the black diffuse shader you want the skulls to turn into. The other material should look like this:
The color ramp is simply to map your texture to its extremes (i.e. skull = pure white, background = pure black). It is then used in a mix shader to make the skulls glossy and give the background a holdout shader (i.e. not rendered, pure black and transparent if enabled).
Next add two more render layers, I called one "mask" and the other "diffuse". Give each one an override material, the "mask" layer should use the glossy/holdout material and the "diffuse" layer should use the pure diffuse material.
Now you can render your scene and head over to the compositor. For the compositing, just mix the main layer with the "diffuse" layer, using the "mask" layer as the mix factor.
Which yields the following:
If too much of the skull pattern is darkening for you just add a Brightness/Contrast or RGB Curves node between the mask layer and the mix node and use it to knock up the contrast of the mask.