Replacing wheat bran in Struan bread.

You can use virtually anything you want. In WGB, Reinhart mentions:

Precooked- Rice, wheat or rye berries, barley, buckwheat, bulgur, couscous, cracked wheat, millet, quinoa, steel cut oats, etc.

Not necessary to be cooked- amaranth, rolled oats, oat or wheat bran, triticale flakes, multigrain cereal mixes, and any type of flour or meal(like cornmeal,etc).

So you get the idea, anything you want or have on hand. Nothing else on hand; use extra of the thing(s) you already had.