Yeah I played around with that one too, but no cigar sadly. And it is as jawz says.. the way lights function is a base functionality, cant just be added in. Just like you can do lots of awesome stuff in 3dsmax.. but it will never get into the game because the game does not support all the texture layers.
The solution I used in my blackreach mod when I wanted it... was a less than elegant placement of meshes to create the required shadows on the existing shadowcasters... mainly just make sure they did not have the option to glow upward.
I guess it would be simple enough to create such mesh with a texture that is transparent, but obstruct the "raytracing" (I forgot how the game calculates shadows).