How about an ultimate where a Mini gets an absorb affect for X amount of damage absorbed, lasting X seconds and it gives +X mana regen, and when it breaks, runs out, or gets dispelled, it gives mana back for 50% of the damage absorbed?
But what would the minion do with all that mana? Your idea replaces the mana transfer remember?
I forgot to mention that the ult gives back mana to all units around him.