[Endoperez]

#noprod already exists. You can have an item that needs Construction 10 to be crafted, e.g. an item that CAN'T be made at all. It can, however, be Wished for.

Currently, Elephans only have two misc slots. The units with three misc slots are specialist magic-users, including Crone and Oracle. Size or Strength requirement might work better, but then we would have Firbolgs with Bear Claw Talismans, Boots of Giant Strength, Stypmhalian Wings etc' flying around with all the cool stuff.
It could be done, but not as an item. Check out Juggernauts. They are very close to what you describe. Construction 5, 7 and 9 should have more spells, which would effectively include making an item which only specified unit/construct can carry. Also, each Behemoth in controlled by a Revenant, but it is only seens in the picture of the unit; there is no revenant except in the description. Similarly, there could be items that only exist in the carrying units' description.