The method of pre-flashing b&w negitive film that I know of, is to preflash in camera, into the scene, with the lens in infinity and at the selective aperture for 5 - 10% of the actual exposure, using a Styrofoam cup, or, in my case, with a Series filter holder, with one or two filter sized sheets from plates installed, depending on the plate's thickness.
You can also just do the math and open-up the lens aperture, while focused and adjusting the shutter speed and I do not see any hang-ups in using a ND filter for a longer exposure, through the Styrofoam layer, say on a Hasselblad, TLR, or other leaf shuttered lenses that have a general top shutter of 500th of a second or less.
This might be a good technique for those participating in the current show me your Holga photography thread.
I'm currently using just one Styrofoam layer, but the do not have any examples to show since I rarely preflash these days, what I'm shooting most.
Cheers