I've thought of this idea, too. I think if you take off the shroud and flip the gearbox so you have counter-rotating tines, then mount totally worn out tines (almost straight), and build an expanded steel cage to put over top of it, you might have something. You'd need to build the cage much like a bagger attachment with a top, sides, and bottom with only a maybe 6 inch slot in the front of the bottom. It'd be critical to get this slot exactly where it needs to be to get an effective toss of the rock by the tines. I think you'd need to extend the front of the basket so it nearly flush with the ground so the rocks have something to bounce off into the box.
You may avoid flipping the gearbox by running the machine backwards with the handlebars over the engine. In fact, this may work better because you won't have the gearbox in the way of the rocks going in the basket.
Dunno if my idea makes any sense, but that's what I was thinking.