A whole estate in Chineham Hampshire built in the late 80's have internal walls of this make up. Its called Stramit board.
You'll end up with a bucket load of straw per backbox. there are channels pre inserted in the stuff at regular spacings & the top of the boards are usually accessible in the loft under the floors as the header boards are fixed to the face edges.
As for a fire risk........the straw I'm sure is fire treated.