Good point. It would have to be full character level, because Elemental Mastery can give you a class feature from a class you don't have, and there aren't enough levels in Master of Elements to go anywhere with it on your own.
So, example, an earth dreamer 14/master of elements 1 with the Elemental Study (fire) feat learns the Elemental Mastery (fire) secret. He gains +1 caster level for fire spells and Ritual of Flame as a 15th level emberhaunt. If it worked the other way, he'd get Ritual of Flame as a 1st level emberhaunt, which is useless because 1st level emberhaunts don't have Ritual of Flame yet.