You can absolutely strap a backpack to the rack. That's how I commute. I've found that Surly Junk Straps hold the bag the best for me. (I've tried bungie cords & cargo nets). Some others here have used other straps with success. I find the backpack/rack combo works better for me than panniers do. If you are only planning on using one pannier, a backpack may distribute your weight better as it will be centered on the rear rack.