Your described method would be imho a little harder to implement ... so if Larian would go that way, i believe we all would eventualy get what we want.
Im' not sure it would be hard. They already implemented Hellish Rebuke. The only thing to add is "toggles" in the variation windows if it make sense and a mechanic to select our target (which is something they should add at least for bless/bane/sleep/...)
Of course it would require a bit of job on every reactions. But they'll have to create something anyway.
