You may be able to move huge stones a short distance using serrated wrack, but you'd have to preserve vast amounts of it in its wet state in order to cover a significant distance. It soon dries out and goes all crispy on you (good for your veg plot by that stage). Like all those littoral zone brown seaweeds, it's a tough old bugger, though not the toughest of them. I think that the logistics of collecting enough of it, then transporting masses of it to place at strategic points along the route, would far outweigh in difficulty any other way of shifting the stones...