Hi Shaun and welcome to the course and forum 🙂
Although we have yet to experiment with coffee chaff (it’s on our to do list!) I have heard a few times of others who have added them to the mix with success. It has a texture similar to sawdust and, like you said, the high temperatures of roasting and the low moisture content will make it a pretty sterile material to use without the need for further pasteurisation.
The fact it is so dry (I think when we looked into it before it has a moisture content of around 12-14%) could present an issue if you add too much as it will lower the overall moisture content of your substrate.
Worth some trials – we will probably test adding some at a low rate (maybe just 10%) and also adding some at a higher rate of around 30%, but to soak it in water first to hydrate it.
Best of luck with the trials – let us know how you get on and we will do the same when we get to it.