I have to say that I am quite impressed with what I am finding on this process. Sawdust or planer chips can be used interchangeably in smaller systems. A certain amount of sawdust could be gathered as waste in some areas, likely for the hauling. As Pete observes, the straw is low density, and hand chopping would never do. And I agree that the Chinese have probably used straw due to being so hard up, and probably using it very crudely in bundles fed into cooking fires.
The straw and chaff blown out of a combine with the straw chopper going will be nearly processed enough to use, though gathering it would require some special efforts. (Or a wagonpulled behind the combine). Way back there were cast iron hand powered straw choppers, frightening but effective small scale machines. Tub grinders handle round bales, that would do. Intermediate power and size choppers can be easily envisioned.
It feels like time to get experimenting.