I don't have a grapple, so that this with a grain of salt as food for thought. I completely understand a grapple for grabbing, lifting, and moving stuff. I would be concerned about using it as a root rake. Here is where my concern comes from. A while back, I had hooks welded to the top of the bucket on my FEL. When I talked to the tractor guy at the shop who did it, he gave me a stern warning. He said the hooks are great for lifting and moving things around, but don't dare use them to drag stuff. He said loaders are not designed to take that kind of uneven horizontal stress, more on one side than another. Given your tractor has enough HP and weight to deal with deep roots, I would be concerned about the root rake hitting a root, rock, or whatever that is solid on one side. I would think this would be similar stress as trying to pull stuff with the bucket hooks.
I may be completely wrong about this, but I would at least look into it. I would be much more comfortable with a root rake function behind the tractor instead of attached to the loader.
Thanks,
Jack