I had long ago joked with other posters that I had committed the X100 specs to memory...if not perfect memory. I love a link to someplace where your remarks originate.
All we know so far is that the <Fn> button can be used to call up a favourite menu function...and default set to ISO choices [thus also user assignable].
I have never read anywhere since Photokina at day-zero that the "convenient command lever" [Fuji] or now called "jog" lever [dpreview] is dedicated to, or locked-step with the <Fn> button.
Supposed the jog lever is so lock-linked, but if within the menu there is a preset focal distance option...or other multiple-choice options...wouldn't the idea still work?
[I, for one, is not prone to frequent ISO changes.]
Zone focusing is really just causing the lens motorized IF sub-system to set the lens at a prescribed encoder position...thereby providing the desired DoF within an f-stop...why wouldn't it work?
See also:
http://www.rangefinderforum.com/forums/showthread.php?p=1505329#post1505329