Asking for Trouble......
What you describe sounds like a blooper. I was reviewing Sail Power by Wallace Ross, and he describes a sail that flies on the opposite side of the boat from the spinnaker.
He said that it was originally developed from a genoa, and that the genoa can be used this way, so I tried it on my prior boat. The way it is to be flown is from the jib halyard, without being hanked on the headstay. Then a short (3' or 4') pendant is tied to the tack and the forestay fitting at the deck. Then the sheet is run through a block on the end of the boom. In order to make this work, I had to run directly downwind (kind of hard to do here the way the Columbia River is laid out in relation to prevailing winds.) The idea is to fly it out far enough from the boat that it stays filled. The pendant at the bottom, and the halyard not peaked let it fly quite wide of the boat. I never made it work well. Your mylar sail would probably work a lot better, my genoa was pretty heavy, and I had a difficult time keeping it filled.
Once you have the spinnaker under contro, this could be a fun way to add some sail area. On my prior boat, it came very close to causing a mutiny, the crew wasn't as excited about it as I was.
So, give it a try and let us know how it goes. It could be fun. It could be asking for trouble......