I checked for rental conditons and I don't know if it's possible for me to rent equipment as they need insurance to cover the rented equipment.
What I have seen and written about are:
Samples from a Leica S2 and a Nikon D3X, this article is here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/38-observations-on-leica-s2-raw-images
My main interest here was aliasing effects. What I have seen that the Leica had superior sharpness except in on or more corners, but I was not looking into sharpness.
I also looked at shadow detail and Nikon D3X was clearly better on that. The images were used with the permission of the copyright holder.
I also wrote a lengthy article about MF compared to DSLR. For that article I was able to use images by Marc McCalmont and Tim Ashley who both own an IQ180 on Alpa and Nikon D800E. Both images were used with the permission of the copyright holders.
I used Marc McCalmonts image to study sharpness, which the IQ180 won by large margin: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/71-mf-digital-myths-or-facts?start=5
The image from Tim Ashely I used for judging shadow detail. Here Nikon D800 won in my view: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/71-mf-digital-myths-or-facts?start=2http://echophoto.dnsalias.net/ekr/index.php/photoarticles/71-mf-digital-myths-or-facts?start=3
I don't think the results would be different if I pressed the shutter release. Using images from other photographers is a good practice in my view.
In the same article I used test images from Alex Koloskov taken with a Hasselblad H4DIII50 and a Nikon D800E. I asked Mr. Koloskov for permission to use his images, but got no response. The only use I made of the images was to check color accuracy on his Color Checker shots. That control was done in LR and using Imatest. The Nikon had more exact colors.
Tim Ashley suggested I discuss more about color and suggested I link some of his images. That part of the article is here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/71-mf-digital-myths-or-facts?start=9
I would add that my article was checked by all contributors. I have made prints from several of the images (using small crops). The differences are in general smaller in print than on screen.
Another article I checked was a comparison of the D800 with a Zeiss Macro Planar 100/2 with a Leica S2 and a Leica 100mm f/2 APO-Macro-Elmarit-R: http://diglloyd.com/prem/prot/DAP/NikonD800/diffraction-mosaic.html
In that case the Nikon/Zeiss was sharper in the corners than the Leica.
I use a calibarated Eizo monitor.
I am in no way adversary to MF. On the other hand if someone is asking about low end MF and second hand equipment I think it is responsible to suggest that they also consider DSLR, and the Nikon D800/D800E is at least on the surface the best alternative of those cameras. Highest resolution, best DR at a very reasonable price. Canon 5DIII is another type of camera more geared to high ISO and action, so I think Nikon is the obvious choice.
If you have the ability to rent an IQ180 and a D800. Could you please do your own independent testing and post the results. If possible people portraits as well as landscapes? Also available light and artificial light such as strobes. When you do your evaluations could you use a Eizo monitor? Also please make a print at 11 x14 and then a larger one say at 44 x 60? I look forward to your results.