Well, the most effective way would be to trigger all healing spells. With that you could also add to them a check for (de)buffs to increase/decrease healing amount.
Else you will probably have to make a trigger which fires when any healing spell has been cast, detect which of the healing spells it is from the given list. Then determine how much you heal additionally the targeted unit.
The problem here is obviously with healing-over-time spells or non-targeted spells like Tranquility. This is where the system would get complicated, as you would need periodic events to decrease/increase health of affected units in same time as when they are healed.
Triggering your own healing spells is imo not really a problem and could save you a lot of nerves. There are about 5 types of healing spells that I can remember from the top of my head:
- instant (holy light),
- chaining (Chain heal),
- periodic (healing salve),
- AoE heals without any target (Tranquility),
- targeted AoE heals (Healing spray) - they work same as Tranquility tho