A quilt without batting is often referred to as a "summer quilt" or a "counterpane." They are commonly made, especially in areas where it is hot and humid. I've made two. One I machine stitched a meander all over it. The other I just machine stitched along both sides of the lines of stitching on the top (like Tom W described). I wash in my washing machine without problems. If it is well stabilized, it should not be a problem. But if it is going to get heavy use, I'd be afraid that the tshirt fabric would start stretching and getting wonky over time and would put a thin batting inside.