Carbon fiber rods, like mattia is talking about does seem to not only make the neck more stable, but it eliminates dead spots you'll find on some necks. Since I started using them long ago, I don't think I'll ever build another neck without them. It gives the neck, better tone, more stability, and what else can you ask for. I also like the idea that once you get the guitar setup done correctly, you don't have to adjust it due to climate changes, etc. At least this is my own experience with CF rods, and you should test it out youself though and make your own conclusions about them.
As far as adding mass to the headstock, I can see how that would increase the sustain also. Very interesting thing to consider. I've seen builders use CF rods on the headstock also, a veneer on top to cover it, I might have to try that on my next build.