I put the cosmetic override devil tail on a mantle of the raven and instead of the tail I only see the raven cloak.

Not sure if this is working correctly.

The cloak is too big, the tail renders below it. Nothing to change about that, I'm not gonna change my cloak render for a single cosmetic item ^^
Also: Try the translucency enchantment. It hides the cloak, but the tail still shows =P