Lighting Question

Have you tried using a Projector actor?
It projects a texture onto a brush or surface.
You can get nice results with using it as it lol, 'projects' onto meshes,like your character...looks good.(animated meshes)
I know it makes a texture but also lights the area too,considering your invisible brush I'd use a ucGeneric.utx texture,one of the glass textures in there,and project onto that.
Another thought is Spotlight actors onto a glass texture

UT99 or 2k4?
In 2k4 you can use a static mesh, deselect bProjectShadow and use an alpha texture for the glass.

In UT99 you could use a solid brush with "Invisible" selected in the face properties (DM-Faith an example). Place a light flare on the underside and a searchlight actor on the top (pointing up), or just light actors on both sides. Properly placing your lights will remove any shadow effects. Also if the "window" does NOT completely fill the opening, it will not cut shadows onto the surrounding BSP edges, and if it does, try placing light actors in this small gap (1-2 UU).