I am stuck at the same thing. Been trying to solve muscles using vellum tetrahedral volume but not happy with the kind of deformation I am getting. Its bending instead of budging up while squashing. Tet fibers sounds interesting, but could not find anything which explains how can we set the fiber direction per point.
I came across this H17.5 features video, if you go at the very end, he explains various ways to set materialW attribute. Not sure if we can use something of it to achieve what we want.