Thanks for you're reply. As I have never actually built one of these things, I value your experience. I like the suggestion about the sonitube. I would need to go dumpster-diving at a construction site to get some. I can see the advantage of using an inert insulating material such as perlite. It probably fills the same role as grog or temper when making pottery. The only down side I see is that I can't locally source perlite or vermiculite, so I would have to go buy it.
I have heard many references to using perlite and/or vermiculite with clay to for an insulated riser, which I'm sure works great. My interest in the bio-char as an insulator is mainly because I have some, and I'm looking for ways to use it. I've built the 1G toucan TLUD stove Paul featured in one of his videos, and now I have several gallons of raw, crushed bio-char. In the video below, Ernie's friends at Aprovecho describe making insulating refractory brick with clay, sawdust, and water, which then needs to be fired in a kiln. The need for a kiln is the main reason I wouldn't just make the brick, because I don't have access to a kiln, and I'm not willing to make a kiln. But, the only draw back from using the Aprovecho recipe for a cast in-place riser that I've heard is that the sawdust takes a long time to burn-out (and that it tends to crack when drying), so the full benefit of insulation is not realized until much later in the life of the heater. This then leads to my question of why not use bio-char instead of sawdust.
I can appreciate the tried-and-true advice to use perlite/vermiculite as the insulator, but something seems off if I have to create demand for a mined material that needs to be shipped to me when I could potentially replace the material from local, renewable resources while gaining the heat benefit from its production. I'm sure I'm not the first to have this question. I'm often disappointed to realize that my great ideas have been thoroughly documented in the 1800's. I guess the real answer is to try it myself and report my findings. That may take a while to get around to...