Best answer I've been able to find for you is
thisBasically it says that because face down monsters are not considered 'Public Knowledge' you cant use Super Poly with your opponent's face down monster.
I think there is a Similar Ruling with Catastor, if Catastor attacks a face down monster that monster is considered to have "no Attribute" until it flips face up. Because the monster flips face up during damage calculation the timing to activate Catasotr's effect is missed.
So based on that ruling I would conclude that because the monster is face down it is considered to have "no attribute" and therefore cant be used to super poly an Omni Hero.