OS supports Natural Armor with both 'TYPE=NaturalArmor' and 'TYPE=NaturalArmorEnhancement' you can test this out in the latest Trunk Build. Amulet of Natural Armor and Draconic Blood Line Level 2.
I've also updated my proposal on Armors on the _exp list. The currently solution allows items to stack that SHOULD Not stack. Plus my proposal should fix our ongoing issue of Archmage Robes in 3e that hasn't been resolved in several years. Might Just need to add an extra 'Armor' Type for the Shield, ArmorShield or whatever, since Shield isn't it's own Type.