I have seen a number of these showing up lately. I cannot speak to the reality of smoking an antler bowl, but as ejames has so aptly pointed out, they are of the same material as hair.
If you have ever worked horn or antler with high speed drills, dremel tools, etc. you are aware of the stink that ensues when the material overheats. I cannot imagine to many things short of raw sewage that would be as unpleasant as a bowl of lit baccy in an antler pipe bowl.
Stem material would probably be ok..... if it is not in direct contact to the fire. Just my opinion :O)
American Indians made their pipes of stone. Many existing examples have wooden stems, ornately wrapped in porcupine,beads,paint or even plain. I have seen them in many types of stone in N. and S. America. Frequently catlinite (argillite (metamorphosed mudstone), a red and or black carvable stone, was and is prized and held sacred by Native People.