Okay, testing done, delayed a little by a herd of reindeer hitting my trap fence.
Iron coudes and kneecops work as intended.
Iron helm, iron spectacle helm, and iron codpiece work, but have the armour protection of the leather used in their fabrication. This makes them very resource intensive and almost useless. I don't know how to fix it, but likely remove the leather from the recipe and they will work. The leather would just be a trim to keep the metal from contact with skin, with the helmets you'd wear a cap comforter of leather or padded cloth underneath. With the codpiece I am not so sure, having never worn one!