I found this method (which is possibly already known by many...) to make sector light level differ on a number of height levels.
This is made using absolutely flat 3d floors (i.e. ceiling level = floor level) with sector contents set to 100% transparency and passability (flags value = 7) (you may also remove all 3d floor textures and flats, but I believe its not obligatory since you make it fully transparent).
Thus, a 3d floor, while being invisible itself, makes everything below have same light level and some extra effects as applied on 3d floor. Since you may use numerous 3d floors stacked one above another, you may get something like on following shots.
Light level with angle (made by sloped 3d floor):
Well, also introducing faded sector effect - in case you did not know its available in Vavoom: