That still won't solve the root problem. There are many different kinds of weapons. Normal guns used by fighters, huge guns used by capital ships, bombs/torpedoes, you name it. So what you suggested above simply won't cut as one-for-all-solution.
To give modders and fredders real flexibility, this really should be in armor.tbl. Since the default is 9% hull strenght when piercing effects occur for non-beams, this value should also be able to be overridden in armor.tbl.
When both of those are in armor.tbl, it gives maximum flexibility over the feature. Especially since there are new sexps in 3.6.11 that allow changing armor classes of hull, shield and subsystems on-the-fly in-mission.
Need an example? A long-range beam artillery strikes a critical hit at a ship that is "healthy", yet the beams seems to strike through the target with piercing effect, this is one-time only effect in the mission. This could be accomplished only with armor.tbl. You can replace the beam with a huge-ass railgun if you want.