I would think that if you attached the tether to the wing tip, if lift increased beyond the weight of the glider itself it would behave like I would think a control line controlled model airplane would and fly in a narrower and narrower cone until the reduced lift from the lower projected area of the wing matches the weight of the airplane, and possibly crash in interesting ways.
Or maybe with the centrifugal force and the low radius that we see here it works out.