Barbed devils have a passive physical attack that involves you getting pierced by their barbs. Possibly if you are wearing body armor and/or gloves, or wielding a two-handed weapon with longer reach, the passive attack may not always hit you. (Polearms should never incur this attack, but the way passive attacks work by default should already ensure this.)
Artifact suit of armor (type is unspecified, perhaps a ring mail or chain mail) that is unaligned and operates like magic armor in the Legend of Zelda. When worn, it drains your gold at a constant rate, and when you take damage, you lose a somewhat randomized amount of gold, in exchange for partially or completely negating the damage. Gold is consumed at some rate higher than 1 per turn, and it is lost for damage at some rate higher than 1 per hit point.
If you have no gold while wearing it, you become either very slow or very encumbered.
Croesus may occasionally generate wearing this armor, along with some gold to supply it. Otherwise it generates like any unaligned artifact.
A headless race, “Akephalos/oi”, which has no head and its face on its chest, so it is incapable of wearing helms, and is blinded (and cannot drink potions) if a shirt or body armor is worn.
Also a dullahan/cephalophore race which Knights can play as. They are also headless and cannot wear helms, but they carry their head as an item and can place/throw it elsewhere. Vision is based on where the head is, so it is possible to leave the head somewhere and have the body moving around blindly in a different area of the map.
Body armor imbued with a monster’s soul in a forge can convey you some resistances of the monster.
Polymorphing while wearing crystal plate mail, leather armor, or iron mail can cause you to merge with the armor and turn into that respective material of golem (glass, leather, or iron), with the armor returning unbroken when you revert back to your normal form.
Add a new sort of body armor that makes a compromise between studded leather and crystal plate mail - less AC than crystal plate mail, but more than studded leather (so around 5 AC), weighs more than studded leather but less than crystal plate mail (so around 300-350 aum), and doesn’t hinder spellcasting at all. Possibly, “crystal scale mail”.
Possibly crystal plate mail’s base AC will have to be boosted, or its weight lowered, in order to avoid this new armor being just better (as it stands, a gain of about 2 AC in exchange for about 100 extra weight is something most players won’t take).
Armor of training: has no AC, but gives you a small multiplier to your experience point gains (and possibly skill point gains) whenever you are wearing it.
FIQhack has added this, but as an object property rather than a new object.
An early-game body armor piece that increases Pw regeneration.
Armor of lifesaving. Works as expected, saves your life but disintegrates in the process; more interesting than an amulet because it leaves you more vulnerable after disintegrating. This could possibly be an (extremely rare) object property.
Fish scale mail; awful AC but grants magical breathing.
Lizard scales, available as a drop from killing a lizard, are wearable as AC 1 body armor. They can be upgraded into lizard scale mail, which has AC 3. This armor protects you from petrification.
