Pre Flashing the film seems like an interesting idea I will most definitely try..!!
However what do you mean with GNDs? Neutral Density filters?
Gradual Neutral Density. The density changes gradually across the surface of the filter. It's used a lot for lowering exposure in bright skies, for example, while retaining detail in a darker foreground.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh