Taking lightmap value from floor?

I've been investigating the techniques Bungie's Halo 3 used to fake certain effects. I found out that none of the shadows from the terrain or static geometry are real-time shadows, and that the objects within those shadows being 'shadowed' are actually just being darkened based on the light of what they are standing on. I'm wondering how I'd write a shader that samples the lightness of what they are standing on? Or would it be better to use a script instead?