Our informal check of the records indicates somewhere around 50 Alembic instruments were produced with graphite necks. We've been using pultruded graphite rods in our 6-string necks for many years. I found a supplier about 12 years ago when I first got interested in stunt kite flying and noticed how nice the graphite rods in the kites were.
The graphite necks we purchased had limitations. If you want a custom neck dimension, you need to buy a new mold. With no truss rods, there is no way to change the relief in the neck to suit differing styles, or to adjust any movement (yes, the graphite moved). Of course you can still make height adjustments with the nut and bridge.