Hyper-tufa is definately something that can be sculpted. The only mold I plan to use with the stuff is those big plastic island faces that you buy at party stores (Hawaiian luau type stuff). I'm going to pack the tufa in one of those and then hang my sculpture on my garage. You have to pack your base pretty hard to give it firmness, and let it set for around 24 hours (wrapped) before adding your embellishments. It's kind of like clay in that you work with it while it's wet (additive), but you can carve on it wet or dry. Did I share my fire grot link - that was my first attempt with working with the stuff. :-)
I haven't checked the links yet, but tell me a little about the molds. Is H-T something that could actually be sculpted? Additive or Subtractive? Any success stories? Failures?