I had been thinking about this as well. So far my research is not bearing positive fruit. I bought a bought at my local big box store. While it is light, the water seems to move through it too fast. I don't know if that is design or cheap manufacturing. all my research into the manufacturing process kind of leans toward "this is too toxic" to put near my food. Perhaps making it myself would give me control over the materials used, but in the end it has proven to be Too Porous to use as an olla. At least so far.
I can not find any information on this. I would assume that hypertufa made at home would be no more toxic then any other chunk of concrete, perhaps less so. But I don't know if it would work.
My terra cotta pot ollas were not porous enough.
I imagine that even if the hypertufa was a little too porous, it wouldn't matter much in a dry hot climate like mine. It should still be more efficient then watering on the surface, rather like a micro drip that wouldn't break down all the time. The only loss would be the self regulating of a terra cotta olla, but that is no great loss in a climate where it does not rain for weeks at a time; the water is usually wanted, and the olla wouldn't be filled in wet weather.