I think there isn't such a thing.
I suppose the easiest way would be to use a Damage Detection System to alter the damage received and deplete the mana if mana is not 0
Can't give you any further help, I never used any third-party libraries.
The issue I can see with just using the event "A unit takes damage" is that, IIRC, the event fires before the damage is actually applied.
Probably tricky to implement that in plain GUI, but this is not my specialty.
This said, although waits are not recommended, I don't think it would be too problematic if you added a short wait in the actions before altering the damage with "Event Response - Set Damage of Unit Damaged Event" and depleting mana. Maybe it would.
The filter to make sure the depleting only happens to units with that passive shield should just be based on a dummy passive ability, and then add the check in the conditions to see if trigger unit has the ability.
Not sure this will work 100% but thats probably worth a try (if you don't want to use DDS).