I also want to do exactly this. I was unable to figure it out, but here's what I tried:
(assuming "male" is a cal3d avatar with a bone called "Bip01 Head")
Code:
import viz
import vizmat
viz.go()
viz.MainView.setPosition(0, 1.8, 1.0)
viz.MainView.setEuler(180, 0, 0)
headbone = male.getBone("Bip01 Head")
trans = vizmat.Transform()
trans.setScale(0, 0, 0)
headbone.unlock()
headbone.setMatrix(trans)
The head doesn't change at all.
my guess (and this is a wild guess) is that at some point in vizard's code they normalize the matrix.