I had a play with the Manfrotto focussing stage, and a reversed 28mm lens on my D200. The subject was a docile Marbled White Butterfly. The image is rather poor, but at least it gives an idea of what can be done. The only way is up!
I've attached the image and a 100% crop. There is some fuzziness near the eye, and I am not convinced that is not an artifact of the lens. I used a Nikon 28mm F2.8 AIS lens at F16.
What I found out is that:
a) DOF is incredibly shallow making focussing very tough even with a rail.
b) The slightest bit of wind makes life impossible.
c) Composition is incredibly hard (see point a).
d) Lighting is hard because the lens is so close to the subject.
e) The lens sharpness at F16 seems pretty good, at least within the small zone of sharp focus. Chromatic aberration is zero.
f) I focussed with the lens wide open. I then had to stop the lens down. This is a right pain because you have to put your finger near the subject, potentially knocking it, and it is hard to see the aperture scale too.
What is really needed is a miniature focussing stage with movements of no more than an inch maximum. Ideally x, y and rotational movements are needed. Something like the bit on a lathe that holds the tool turret would be ideal, but smaller and lighter.
Or use flash and dispense with the tripod ...
The more I try this the more I admire the fantastic 2:1 close ups on some web sites.