If Mirrormade copies an Aura this way, you choose what the Aura will enchant just before it enters the battlefield. You can't choose any permanent cards entering the battlefield at the same time as that Aura. This doesn't target the player or permanent it will enchant, so an opponent's permanent with hexproof may be chosen this way. The chosen recipient must be able to legally be enchanted by the Aura, so a player or permanent with protection from one of the Aura's qualities can't be chosen this way. If there's nothing legal for Mirrormade to enchant, it stays in its current zone (unless it's a spell, in which case it's put into its owner's graveyard).