suggestions:
as has been said, ears could be head slot
eyebrow for the eyes
my thought is that some piercings could be unique home brew items:
mouth / lip ring could be an item that makes a tougher dc for enchantment spells for smooth talking casters
nose ring could be a + to save vs inhaled poisons or to extend underwater breathing
ear ring could enhance hearing based perception
nipple rings could be a charisma boost. requires to be visible. A great boon for un armored such as scorcerers/esses
belly button as said would be belts, thoungh on me personally, I have none. i was hatched [truth: surgically removed-life saving procedure, not cosmetic!] I would not get a ring there.
as for 'other' as lwaxy brings up, a cursed item: changes genitalia to opposite gender, just not the rest of the body. caused by the ACTUAL item meant to change gender or general appearance as a minor polymorph self spell.
I would not get a ring there either, too risky.