Here's how I would do it:
For left-column gear (Head, chest, hands, legs, feet):
Fending, Maiming, Striking, Scouting, and Aiming gear can be displayed by all Disciple of War jobs and classes.
Healing and Casting gear can be displayed by all Disciple of Magic jobs and classes.
Crafting and Gathering gear can be displayed by all Disciple of the Hand and Disciple of the Land classes.
Class/job specific gear (I.e. rewards from completing class/job quest lines) remains restricted to its given class/job.
For right-column gear (Earring, necklace, bracer, rings):
All gear can be displayed regardless of class or job.
For weapons and off-hands:
All restrictions remain as they currently are.
Here's a mockup tooltip I made (Note the change to class/job restrictions, directly under ITEM LEVEL 510):
