One technique for adding long-lasting moisture and richness to homemade bread is to recycle old bread. It was a technique apparently used regularly in Eastern Europe. Stale rye bread was soaked in water and then added as a mash to a new sourdough bread.
But why not try other kinds of stale breads soaked in something other than water? The boule below includes a leftover wholewheat loaf soaked in my homemade soy and rice milk. It is leavened with my Meadville white flour starter and also has a cup of spelt flour.