I'm quite sure the vision height should be the same as the model height - it makes sense - so shouldn't that work if the model isn't animated? Besides, I don't think there are any animated pathable destructables in wc3 so it's fine for most people.Element of Water, no. Destructable Z is based on the model, so you could even have a variable Z if the model is animated. That native returns the vision height which is used to tell if a unit should see behind the destructable.
If GetLocationZ does not return destructable terrain height, then there is no way, sorry.