Dr. Ashby and I have been playing with footing carbon shafts for awhile now, in fact he's in the bush testing some as we speak. It appears many of the broadhead failures where bone is encountered in realty start as shaft flexing/failure just behind the head. The best footing would be shaped just like they are in wood arrows, transition from stiff to weak over 4-6". Just running something inside (or outside) that stops abruptly just moves the weak area from one spot to another.
A combination of aluminum and or carbon could be used and done in a telescoping way to transition the stiffness and would make the shafts even toughter then they currectly are. There is a company called Aerospace Composites that makes carbon tubes and solid carbon rods of various sizes if someone wants to experiment with it. Their web site is:
http://www.acp-composites.com Something we've seen too using existing "tuned" shafts, the added weight up front doesn't effect tuning much cause it also stiffens the shaft for a net trade off. Helps in getting very high FOC balance which is a good thing too. Something to play with that appears to have huge benefits, if for nothing more then stump shooting!
.....O.L.