GLU_FILL is just about filling polygons not the inside of the volume.
EDIT:
I think this is possible to obtain this effect using a shader. I have never tried this, but I am thinking that you may give the clipping plane equation to the shader and determine in a fragment shader if a fragment is on top or underneath the plane. In the first case you do all lighting calculations as if you were on the plane (using plane normal, etc…)