There’s a number of ways this might be done, some in the Scene and some in post-processing, and all depending on the renderer being used (Blender Internal Vs. Cycles for example).
If you want to filter your existing Lamp in some fashion, mainly luminance, you could convert your video to greyscale and use it in the alpha Influence channel of a Material w/Texture (Image or Movie) applied to a plane set in front of the light. This would act a bit like an animated neutral density filter.
Lamps can be given a Texture that they then project. It’s not quite as simple as that but you can see the possibilities.
With Indirect Lighting (Blender Internal), simply applying the movie as a Texture to an off-camera plane can act as a supplementary light source, complete with color variations per the imagery. I’m using this extensively in parts of my show reel and it looks cool, very interactive in the Scene.
In the Compositor the movie, either greyscale or color, could be used to modulate the Factor of various nodes, and with Render Layers, or your imagery rendered in layers, it could be applied selectively to particular elements in the Scene.
Or, you could simply animate your light sources, synched to the BG movie.
Which way to go depends on the look you want.