Upvote:2
I can't really give more info than what you already said, but based on what I've learned about them from the links you gave here's what I think.
The legend says they come from a knot made of snakes using their saliva . Since it's a natural stone, found and not created, it would feel off carving anything into it as it is said to already be an object of great power. Plus I didn't see any mention of carved ones.
If you want my opinion (even though there are not many facts backing it up), you should leave them natural. Maybe practice your carving in different amulets, but from what I've gathered hag stones were used without being carved. Although it would definitely look cool.