This is the 90mm without the bayonet,
See the bayonet conversion here, http://leitax.com/conversion/leica/APO-Summicron-90%20ASPH/index.html
Digging into the F-mount specs, all we need is to hold and release the diaphragm, while indicating the aperture. Two options for the latter, either we chip the lens as indicated by Bjørn Rørslett, or we develop some ugly external meter coupling. We leave this for later.
The real improvement for us to forget about stop-down metering is to manage the diaphragm. To hold and release it, two problems are to keep us busy. First, the rotation sense is opposite to Nikon's (not the either way, it's them who turn to the opposite side), and second, the radius of the Leica diaphragm control mechanism is bigger than the one in the Nikon mechanism.
There is some space between the rear lens group and the bayonet, and it seems to be more space inside, I have not disassembled the lens further. Before that, I´de like to explore possible mechanic alternatives. If we can solve the control with a simple mechanism, it seems straightforward to design an epoxy resin piece to hold it, the cpu contacts block, and to fill the empty space to avoid internal dust.
Probably simple levers with some spring to make it fly will do. Ideas?
Follow the discussion here.