That's sounds like a good idea. However, you will still need a damper of some sort either manual or a back draft damper, like in a bathroom. The problem is the cheap backdraft dampers are all round and also require that they are not turned upside down (sideways is OK also). It might be better to have a round duct in the attic with an elbow to it so you can install a backdraft damper sideways. Then run a square plywood duct in the laundry room to the dryer.
I actually like the idea of a square roughed-in duct in the laundry room. It won't have the mickey mouse look to it of an exposed round duct.
