Great find Obviously more porous as you pointed out but they moss up quickly with me. I now put some plastic tubing over any anchor wires in contact with the tufa as they cut through and crack easily.
I saw that the Pacific Bonsai Museum used a piece of plastic (a carpet protector for your rolling office chair) to protect a historic wood slab during a report. What about lining the inside of the pot with some plastic sheet? Yes, the roots would grow into the tufa at drainage locations and perhaps the top edge, but it would be much easier to remove. You could also permanently seal the inside with an epoxy resin. If the tufa is very porous it might take several coats so I would include a layer of very light (0.7-1.0 oz/sq yd = 24-34 g/m^2) fiberglass cloth in the first coat to cut down on the amount that soaks in with each additional coat.
Yes I was thinking about the possibility of lining it with something but I hadn’t thought of the coating. Thank you for your input that’s helpful. Ryan said in the Q&A today that he was going to take a look at it because he wasn’t familiar with pumice pots for bonsai. So check back if you’re interested to see what he says.