The asians have put clay in their keys and baked them.Show Image(http://www5f.biglobe.ne.jp/~silencium/keyboard/jpg/realforce/rf106key4.jpg)
That amount of material would seem to be a little excessive to me. Essentially you want to bring the keycaps up to par with vintage Cherry caps, not increase moving mass to plus infinity.
Choice of material is tricky... high density and good internal damping would be useful.