Actually it does work. Hexproof only prevents the creature from being targeted, and auras put into play (not cast, which is the important distinction here) do not target. When the aura enters the battlefield you attach it to a creature of your choosing. Now, if you cast Pacifism then it does target and you couldn't legally target Thrun with it.
Quote from Comprehensive Rules 303.4 »
303.4f If an Aura is entering the battlefield under a player’s control by any means other than by
resolving as an Aura spell, and the effect putting it onto the battlefield doesn’t specify the object
or player the Aura will enchant, that player chooses what it will enchant as the Aura enters the
battlefield. The player must choose a legal object or player according to the Aura’s enchant
ability and any other applicable effects.
Pretty sure hexproof falls under other applicable effects.
Pretty sure hexproof falls under other applicable effects.