Thanks Richard!
I've considered using independent pins or pins that are a part of the back of the inlay.
When you say "undercut" does that mean the edge goes in to make a slight inverted lip to catch the edge of the inlay? ... this is what I imagined.
Either that or possibly the inlay could have a bevel on the top edge and the horn could be heated and pressed down and over that bevel to lock the inlay in place..
Just ideas, I realize without the knowledge I could be commenting on something that is already well known.