I had never heard of hyper wattle before, so I googled it. I think it could work. Since you are talking about using it for infill, you could probably stiffen it by corsetting it between light poles that connect, or at least butt against your wooden structure. I'm pretty sure that would give you a much stiffer wall and allow you to infill larger areas between major wooden supports. It would also automatically help keep the wall straight and would not be too much work. Of
course the wall is going to be thinner than
straw bale, with less insulation. This system looks totally doable by those of us not cursed with an overabundance of muscle mass.
The clay slip will make the straw much less flammable and the straw will be permanently encased in fairly thick, flameproof coating.
I have a brief story about stray insulation. I was an airforce brat and my dad was stationed in Germany during the mid sixties. We were living off base in an old two story house (only heat was in the kitchen). Anyway, my brothers and I were acting like little barbarians fighting and wrestling in a bedroom and accidently knocked a good sized hole in the plaster wall. Behind the plaster (mayby an inch or so thick) was clean, bright, yellow straw, packed tight. I don't know how old the house was, but the electric wiring was outside of the plaster, (in painted conduit I think). Seemed old to me at 9 years old. I remember, after my folks gave us hell for breaking the wall they stood there and discussed the insulating properties of straw, the flamability of straw, and how the plaster had protected the straw. I, of course, hovered in the background taking it all in. Seemed to me that my folks were impressed with the system and didn't seem to worry about the fire danger. The
land lord came over and patched it. Maybe he had raised boys, because he didn't seem too upset about it. So I'm guessing that using straw as infill has been around for a while, and not always using a clay slip even (although I think the clay slip is a great idea for fire retardant).
I mixed fire retardant for a retardant bomber one summer years ago. Even if it dried after being dumped on the plants the layer of dried powder (we used fertilizer) had a definite made the materials much more flame resistant. Seems to me that the clay slip would would similarly, maybe better.