Nothing can really replace carbon fiber because it doesn't expand and contract like wood. It is also light and strong which is what you want in a neck. But if you want hard and dense, Ipe would be a logical choice and would be resistant to bending. This stuff is hard as nails and difficult to work with. Will destroy the edge on tools in no time. Here are the stats:
Janka Hardness: 3,510 lbf (15,620 N)
Modulus of Rupture: 25,660 lbf/in2 (177.0 MPa)
Elastic Modulus: 3,200,000 lbf/in2 (22.07 GPa)
That said, I still don't use a neck reinforcement and have not seen bending. But that doesn't mean it can't happen down the line.