Just finished having a dusk discussion today! I agree that it is a dangerous time and the quandry between NVG or day is very difficult.
The dusk period and resultant skyglow issues are so variable as to defy any attempt at ANOTHER prescriptive restriction or rule. One outcome based rule of thumb that we are going to trial as an SOP is as follows:
"IF the NVG image is washed out AND you feel you have to go in unaided AND you want to turn on the searchlight to help - then you should postphone the landing until dark enough for NVG"
What we are saying is that if the goggle image is too washed out due too much skyglow illum, then you would want to go in unaided. If you make that determination and then subsequently realise you want/need the searchlight on to help pick up detail: postphone landing until it is dark enough for the NVG image.
Obviously, if faced with this situation, an approach away from skyglow was not a viable option (due wind/terrain/etc).
Whaddayareckon?