This would be *trivial* to introduce code-wise. During the penetration sim, simply direct HEATPenetrators to use HEATPenetrability instead of bulletPenetrability. If the projectile has no special property, or the target BISURF has no HEATPenetrability defined, use bulletPenetrability by default. In pseudocode:
if (projectile.HEATPenetrator == 1 && target.HEATPenetrability != null)
penetrability = target.HEATPenetrability;
else
penetrability = target.bulletPenetrability;
That's all! This way:
1. No existing content gets broken.
2. New armor & CfgAmmo can be added at leisure.