How about a sword +1 vs. armored opponents? The game already does this with the axe of hewing (+2 vs. shield using enemies). So consider the following ready to use weaponry:
ELVEN BLADE: Non-magical, but adds +1 against plate armors, although only of the regular sort. These are highly sought after, being given as gifts only and never sold.
MAGIC SWORD +1 VS. ARMORED OPPONENTS (including chain). This includes magical kinds, excepting perhaps those, like champion mail, having non-standard abilities.
ENCHANTED SWORD +1, +2 VS. ARMORED FOES (chain or plate mail), the latter bonus working only against magical kinds.
|Armor piercing blades are deadly, |
but only against fully armored opponents...
Obviously, this opens the way to special armor types:
DWARVEN PLATE: Non-magical, but still completely resistant to armor piecing attacks, including the thrusting move from the Fear!, Fire!, Foes! supplement (if used).
Note: The referee can assign value per their campaign...
Armor penetrating weaponry is easily achieved using these simple mechanics. And, obviously, the referee is free to fabricate others, with no limit to what they might do, so keep your blades sharp!