Rob,
I guess the mechanical link part should be qualified in my case to include all lenses with an aperture ring. For those not familiar with the Nikon range then I should say that there are two lens types within that range. Normal ( ie old style with a seperate aperture control ) and G type.
G type do not have an aperture ring, and apertures are set from the camera command dial, so I guess that makes them similar to the current Pentax range. Pentax still retains mechanical diaphragm linkage hence its compatability with a shed load of older lenses.
So excluding my one Nikon G type lens ( 12-24) I can mount any of my lenses M/F or A/F on my ancient bellows and have full aperture viewing. I can also reverse any of those lenses and again have full aperture viewing. My bellows is auto but in the Nikon range you can have massive extension through bellows extensions, in which case lens aperture control and camera triggering, is via a twin release.
In the Nikon system I am not aware of any means ( accessory ) of controlling the aperture of G type lenses once the link to the body is broken, so the mechanical link to aperture within the lens, is system compatability only. From a bit of a net trawl of images of them, it appears that the current range of Pentax lenses are the same as Nikon G type in that aperture is controlled through electronic connection to the camera body. No doubt Duncan or Ian will correct me here.
As far as I can see Canon has gone the all electronic route so requires contact between lens and body electrical contacts to control aperture and lens stop down. I notice Novoflex do an adaptor for maintaining the link on a reversed lens for Canon but that is listed as being 299 Euro's, also auto bellows units at 549 Euro's. Canon may do similar but I have been unable to find any info.
As always I stand to be corrected.
Don