Add three new attributes to armor: Bonus Coverage and Bonus Bashing and Bonus Cutting Protection. If a coverage check for a specific piece of armor succeeds, it then checks for bonus_coverage>0 and then uses the same result to check if the bonus coverage was hit as well.
So a person is wearing a 90% torso armor with 3 bashing, 3 cutting protection. It has bonus coverage of 10% (not reliable) and bonus bashing 3, bonus cutting 6.
If the person is hit in the torso, the game rolls an 85 out of 100 and so the torso armor is struck, the roll would need to be 10 or under to get the benefit of 6 bashing protection and 9 cutting protection from that attack, gaining the bonus protection.