are you in a high LP area?
i think one problem with the idea of using a real L image is that the bandwidth of the L image is enormous compared to the NB image. so you're going to pick up stuff that has been filtered out of the NB images with the L and if you have any sort of LP going on the L is really going to contain a bunch of extraneous light, for which you have no color info in the NB channels used to make the RGB...
rob