Wanted: Cycles "Shadow Only" Lamp - Not a simple "shadow catcher" question Hey BSE, thanks for reading!

I'm working on a project in which my scene is brightly lit solely by world surface emission.  This creates a stark, bright "ambient light" or "global illumination" look, with subtle shadowing in corners and tight spaces.

This was exactly what I was asked to achieve and everyone's happy - until the project lead asked for interesting shadows.  

My task now is to:
1. somehow introduce shadows that can be cast by a lamp in a normal, intuitive way.
2. force those newly-shadowed areas to be darker than they should be given the ambient light.
3. ideally, prevent those lamps from adding any light to hit surfaces.

The problem is two-fold:
1. The lamp which casts these shadows should not necessarily add light to the objects it hits.
2. The shadows cast must be much darker than the ambient temperature of the shadowed surfaces.

I'm convinced there is a clever way to achieve this in the compositor, but I'll be darned if I can figure it out.  

Can someone help me learn to achieve this illusion?  A push in the right direction might go a long way.
My humble thanks,

Edit: Added Screenshot: http://imgur.com/kBekWjy
Use two different render layers

Put the "lamp" on a separate layer and Exclude that layer from the render by clicking on the corrisponent box in the Layer panel.

Your scene now should be affected only by the environment's light (and eventually other light sources, but not the mentioned "lamp").



In the second render layer, uncheck the Use Environment box to remove world's shader influence. The result is a scene lit by the lamp only. Enable this this time the Shadow pass in order to acquire the info about where the shadow fall.



Finish by color mixing the Composite pass from the first render layer with the Shadow pass from the second with Multiply option. Use the Factor slider to adjust the influence of the lamp's shadow in the final image