You're welcome Owly. It's a rule that I stumbled upon a while ago, and find perfect for E6.
Da'ath : The wound idea for duels is a very interesting one ! Is it the second part of the Vitality/Wounds system, the Injury one, or something custom you made ?
Ninja : Then only allow the rule against criticals and attacks that would drop you below 0 hp, and the problem should solve itself. In most cases, that's when you'd really want to do it anyway.
Kirth : Total damage vs shield hp would work too. Though lazy as I am, I would probably find that harder to track than just comparing enhancement bonus. :D
More seriously, my odea comes from the fact that I'd prefer a magic shield (hopefully with a lineage) being vulnerable to destruction only against things like "Sundarr, Cleaver of Gods" the mythical axe, but not a lucky roll on a crit x3 by a brutish ogre.
And good find Azoriel. I completely forgot about this feat. It does almost the same thing, indeed.
Sometimes I regret the fact in 3.x/PF that all ideas that would make good optional rules get sneaked in as feats, and then inevitably end up never used because of better feats mechanically and the limited amount of feats. But that's another debate.