Don't know - just a thought. I don't shoot digital, so this suggestion may be completely useless - but what is the auto-exposure range (in secs) of your camera? If you piled a bunch of ND filters (or even an unexposed but processed piece of transparency film) in front of the lens, would the exposure time be great enough to achieve the effect? (May get your exposure times down to 10 or 15 seconds...?)