Thanks Sombre! I don't know where I got that idea from, about having to actually hit the shield. But it's totally wrong! I tested with attack-30 mr-1 units against a def-10 unit and they lost an eye on every single hit. I tested with attack-1 mr-1 units against a def-20 unit and they lost an eye on every single miss.
So every melee attack on a unit who is wearing an Eye Shield always causes eye loss if the attacker fails his MR check.
Sorry Slobby, my original post was wrong.