There's a trick to this. Dress up the mannequin. Go to the retainer it's associated with and remove the gear from the retainer itself, and your mannequin will keep the gear equipped without it taking any slots up.
You can do the same thing with glamor chest and glam plates. You can restore all the items in a glam plate but still apply it as though you still have the items stored.
On another note: I'm DESPERATE for weapon rack furniture. Please, Square Enix.