If it were up to me, I won't experiment with alternatives on VFTs, for VFTs, Dried LFS or Peat with some sand or perlite is safest. But for others you can try other alternatives.
I think that the reason why it is "sterile" is because of the very high acidity of the medium. however please don't think that it is really sterile, we just mean that most molds, fungii and bacteria do not thrive in them... but it doesn't mean it is completely free from other microorganisms.
TTFN
Arvin