That has been a common request. I may end up adding a binary (just off or 5V) moonlight output eventually, but I am hesitant to add too much complexity. In many ways, the code is already more complex than I want it to be -- the plan was to have a barebones lighting system that was simple enough behind the scenes that by looking at it, reasonably techie people could see how to add their own things (CO2 control, temp, etc) where needed.
In any case, if I had a single moonlight I would just rig it to be on all of the time. Much easier and you won't notice it on during the day anyway with the normal lighting active.