Hey Reno --- Lava rock should make an excellent base for applying hypertufa or just about any cement based mixture. Cement relies on either a chemical bond ( as in liquid bonding agants...aka: "glue")...or a good mechanical bond. The porous lava rock should provide a very good mechanical bond.

If you are going for a faux rock finish on a waterfall/pond, I would highly recommend using a modified conventional concrete or mortar mix rather than the 'Tufa. It will be as easy to work, but much stronger and less prone to water permeating the material. A healhty amount of latex admix is advisable. It will make the mix stronger, bond even better and far less permeable. A big issue if you live in a hard freeze zone.

Good luck. It is a very do-able project, just continue doing a little homework before you invest a lot of time & money. And don't hesitate to conduct some small scale tests to see how a given mix works before you go whole hog.

All my best,
Tango88