You can use leftover cooked grains (like rice and oatmeal) in muffins or other quick breads, by running them through a blender or food processor along with other liquid. Then add to your batter as part of the liquid portion.
A week ago, Sunday, I made a large batch of cranberry-almond-apple oatmeal, for breakfast. There were leftovers, which I worked on myself all week long. By Friday afternoon, I realized that I'd better use what was left, before it developed mold.
So, I ran it through the blender along with part of the milk for the muffins that I was making.
Voila -- Ginger-Pear muffins (with the blended oatmeal as my secret ingredient!).
In the past, I've also used plain, cooked rice, this way, in muffins, and seasoned rice pureed with milk, as liquid in savory biscuits. A plain or sweetened, cooked grain will work in banana bread, as the small amount of milk usually called for, or, as liquid in a coffee cake.
It works very well, and the oatmeal or rice is practically undetectable (with the exception of an odd piece of nut, here and there, when using a grain cooked with chopped nuts). This trick works best in a muffin or bread which is going to have a small amount of texture, anyway, such as my Ginger-Pear muffins, with the chunks of pears baked in, or, in the case of using cooked, brown rice, adding to a batter for a wholegrain muffin.