MoP cloak effects should be transmogable - I personally would love to be able to see the effect as either attached to the cloak or unlocked as a seperate option akin to
WoW! Thoughts! — On Earning My Legendary Cloak | Mash Those Buttons
Okay...So the Wardrobe wrathion gives you, allows you to have the effect of the old MOP legendary cloaks...And we still can't have ours show up with our transmogs. : r/wow
Visions of N'Zoth's Legendary Cloak - Upgrading, Stats, Special Procs - Wowhead