Manasteal as a Sorcery keystone or rune?

Title, basically: what if there was a rune that restored mana proportionally to the damage dealt to champions?

Imho it would be great on poke mages (they'd get rewarded for landing spells on the enemy champions instead of passively clearing waves), and could be good on battlemages too (they wouldn't need to build more than 1 mana item, or maybe none at all).