Hello Julian, Quite frankly, I have never heard of anyone using diatomaceus earth in any part of a
rocket mass heater. As serendipity had it though, there was an ad at the top of the page advertising it....here is the link :
diatomaceus earth info
Have you read the Rocket Mass
Heaters Builders Guide by Erica and Ernie Wisner? Soooo much information on rocket mass heaters that it is a wonderful resource not to have.
Burn tunnels are often made from firebrick, ceramic fiber, refractory materials made into a mold, perlite and clay...etc.
Perhaps if you give us more info on what you would like to do, we can better point you in the correct direction.