I could imagine the rubber plug damping vibrations on something like a guitar string.

Although, perhaps one would be better off filling the space between the head tube and steer tube with something like synthetic quilt batting.

But, I have to agree with [MENTION=136384]unterhausen[/MENTION] that the high frequency low amplitude vibrations carried through the tubes likely isn't a big problem.

It is quite possible that material selection would also affect those high frequency vibrations with higher strength steels carrying the vibrations, and softer metals simply dissipating the vibrations.

A couple of summers ago I got to ride my bike on a road that had been ground for repaving. Whew, the whole thing wiggled and jiggled. I had no idea I had so much frame flex!!!