Honestly, the first thing which comes to mind with respect to a magical "Armor Piercing" attack is, why bother?
If someone has some type of armor they're wearing/using, instead of making an attack to 'pierce' the armor, why not just make a type of attack the armor doesn't protect against?
While the RAW doesn't specifically mention different types of armor, in the real world there are indeed different types of armor, with the different types designed to protect against certain types of injuries or against certain types of weapons. A good example of this would be the different types of bulletproof vest. The name itself even indicates what type of weapon it's designed to protect again, so someone wearing a bulletproof vest isn't going to be any safer from being stabbed or hacked at with an axe than someone not wearing a bulletproof vest.
If somebody is wearing a bulletproof vest and you want to attack them with an Evocation, a fire blast or lightning would work.
In the case of magical armor, like that worked by the Leanansidhe in Changes, I don't see the point (or balance in) allowing a magical, armor piercing attack. For one thing, armor doesn't make it harder to hit, it just reduces the amount of stress taken from a successful hit. For the other, while armor does persist if/when overcome unlike a Block, armor also has half the value of the number of shifts put into it, again unlike a Block.
-Cheers