I discovered, much to my delight, that the incredibly useful Eye AF function of the mark II cameras can be assigned to the focus hold button on native lenses which have them. I find using this button is so much easier than using one of the camera's function buttons.
Makes me wish all my lenses had focus hold buttons.
(Maybe everyone but me already knows this but I was excited to figure it out, so thought I'd share it.)
You'll find the setting on the second page of the 'Custom Key Settings' menu