The Army does not restrict you from flying NVGs for any illumination conditions currently... IIRC it would only elevate your Risk Assesment level and require a briefing from a higher ranking commander to OK you to fly (a practice done on every flight). That being said, I've flown NVGs on many, many zero-illum nights in featureless environments in both training and actual operations, and found that while it is a huge help to have the goggles, the missions we performed would have been able to be performed unaided, with the goggles simply allowing a greater safety margin and more efficiency of flight (sans of course the blacked-out combat type missions). Goggles aren't as much a tool to allow you to fly when you could not, but rather they allow you to fly safer when you would have already been flying anyways.
Besides, when you turn even the dimmest landing light on at low level, you create your own moon... and then some.
-Mike