Don't know what kind of room you've got on the backside where the nut is, but if you can get vise grips on the other side turn it as if you were tightening the bolt. The rust on the back side of the bolt is probably what bound up the bolt coming back through the nut. Since there's no head any more, try to tighten back in all the way through the nut. You may be able to drill and tap a small screw in there too and as you tighten the screw, it may get the bolt turning (again in through the nut) and out the other side. It might work.