How you do it is use the texture again as a graduated transparency map. It'll work. (Did for me,
at least).
I was just about to refer you to the orbiter forum for my pltex mini-tutorial, but the forum's down...
Anyway, load up pltex, choose to use your night bitmap as the main map, choose yes to creating a
map, choose graduated, and then choose your night bitmap again as the transparency mask.