Necropotence vs Ashiok, Wicked Manipulator does work as it should. The facedown cards should not be able to be used for milling someone out. That's a lame bug to win brawl games with.

406.3a A card exiled face down has no characteristics, but the spell or ability that exiled it may allow it to be played from exile. Unless that card is being cast face down (see rule 708.4), the card is turned face up just before the player announces that they are playing the card (see rule 601.2).

109.3. An object’s characteristics are name, mana cost, color, color indicator, card type, subtype, supertype, rules text, abilities, power, toughness, loyalty, defense, hand modifier, and life modifier. Objects can have some or all of these characteristics. Any other information about an object isn’t a characteristic. For example, characteristics don’t include whether a permanent is tapped, a spell’s target, an object’s owner or controller, what an Aura enchants, and so on.

202.3. The mana value of an object is a number equal to the total amount of mana in its mana cost, regardless of color.