No real performance advantage, but it sure would look different, and that's what you're after, right? The main problem I can see is making sure the ST is strong enough at the seatcluster. A regular ST would be too thin, but I guess that's a moot point as I doubt you could find a ST that long in the first place. How are you gonna attach the seat anyway?
So, you can get a straight gauge .035 tube or an .049 depending on your weight and the overall length of the tube above the seat lug. Should be plenty strong then.
Cool idea either way.