I suspect the current best bet for fine dust would be a shop-vac sized
cyclone (either the Oneida or the Clearvue) then a shopvac with a
Cleanstream (water-washable, HEPA) filter.

I've had a Cleanstream on my Ridgid shop-vac for a couple years now, and
really like it. It clogs up with lots of drywall dust but the exhaust
air stays clean.

I'll second that - I have a HEPA (Cleanstream IIRC) on my Ridgid
shopvac also, and it filters VERY well. A lot of fine dust builds up
directly on the filter, and I usually just shake it out inside a
garbage bag when I start noticing a suction reduction. (I made a
rhyme...) Once in a while I rinse it under the tap and kind of unfold
each pleat to let it really clean up.
Re. the Oneida vs. Clearvue mini cyclones, have you taken a close look
at either one? Do you think they're much different?
Thanks,
Andy