The list of users to pick from would be the same list as on the assign roles page.

The table would look similar to the table on the define/override roles page, with the same sort of search box. Each row would have the capability name and a column of 'Yes'/'No's showing the result of has_capability.

The Yes/No would be a link to a pop-up that shows the full table of assignments and overrides, as per http://docs.moodle.org/en/How_permissions_are_calculated. See MDL-16965.

If the user has moodle/site:doanything, I may add a third column to show the result of has_capability ignoring that.