I acquired a used Stoneskin Torque for my fight against Maat (MNK), and imo it isn't useful at all during the fight. First, yes: the enchantment does stay on after you've unequipped it and re-equipped whatever it is you wear in that slot (Spike Necklace for most melee).
The caveat is this: the enchantment wears off in about 30 seconds. When I faced Maat, I used both this and a Blink Band. The shadows from the Blink Band were subtracted first, and by the time Maat eliminated my second shadow, the enchantment from the Stoneskin Torque wore off.
For melee jobs preparing for Genkai 5, a Blink Band is all you need. Maat, even though he uses Hand-to-hand, attacks very, very slowly, and I'm positive he couldn't eliminate your shadows fast enough for the Stoneskin enchantment to ever be tested.