We tried to do exactly that as you have pictured. We even set up a version using cheap blue tarps with noodles held in by clothes peg and got the local canvas place out to have a look. We were very disapointed when he said, it is possible but i dont want to do it. He was the ONLY canvas place in town. (And people wonder why we dont buy local!). We got the Sh!$t's and gave up after 2 years of me trying to figure out how to do it ourselves with tarps and noodles ( we even tried dome tent poles crossed over and had to be attached in the corners), then just bought a solar blanket recently (we havent put it on yet).
But i do think it will work. Our slight variations to yours was to attach it to the tent permanently, making the guy ropes not necessary, using the existing tie downs on the roof of the tent in the corners, we were going to get additional points sewn into the tent on the sides in the middle to keep the top tight and stop it flapping.
Good luck and let us know how you go.