I think horn is probably one of the easier materials to add inlays - here is an example - it was my first attempt using three different forms of Mother of Pearl.



I used router bits for the holes then used glue to attach a piece of MOP to the other end of the drill - then turned the discs on a rotary sanding disc. They were inserted using CA and finally sanded/polished flat.

By the way, it's much easier to buy pre formed discs - they are available in lots of different sizes so I used them on combs that I made for my grand-children at Christmas.

I've yet to try any other forms of insert apart from round pieces but, like you, it's something I'm thinking of when I have more time.