I think you will find it is calculated on HEAL rather than on CAST, which means when atonement heals it will also proc cleave. The only question is does it work off absorbs. If not then this trinket will not interact with 70% of a disc priest's healing.
For holy this is not a problem because everything is a heal, so this works off the secondary procs (EoL) themselves, which means it is also boosts our mastery as much as it does other spells.