it would require to be a spell instead of an ability to have no mp cost and instant, since it would also need a separate cooldown.
Instead i would prefer something like a conversion of heal into shield for the next healing ability used, kinda the opposite of emergency tactic.
Like "next healing ability will cost +50% mp, and sacrifice 25% of its healing output into shield", with the addition of not being able to stack this with either Ast and Sch shields.
That would make whm able to make some mitigation and still be into his "aspect" of mp management and overhealing gestion.

Ast and Sch are both able to use some 650(benefic II)-1200(ET+adloquium crit)+ potency heals, why couldnt be have a 100(SS+cure)-162.5(SS+cure II) potency shield like that ?