@Vesh I think I found a way for Soraka to keep her mana infusion.
As you already said it's weird to put it on either of her AoE, but why not put it in the obvvious place: with her Astral Infusion. Of course you can't put both effects together, because it would be a waste if you don't want the other effect.
But how about using something akin to Twisted Fate's "Pick a Card"? Activate the spell twice (or keep pressing with smartcast) to switch from health infusion to mana infusion.
Additionally, you could also put some "on enemy cast" portion back on the Infusion. Considering Soraka has no direct damage spell otherwise.