dtd24,
I don't see a big problem with the basic concept. As mentioned, in order to use a stock hydraulic cylinder you may have to mess around with the pivot point locations is all. And like Jim pointed out, the jaw should open more so it won't get in the way when you are not using it.
What concerns me is the geometry of your grapple jaw. Specifically, the shape where it comes backwards like that, and the sharp angle/corner in it.
With the jaw going back like that, as it closes onto "stuff", it may not "drag" or "roll" or push the shrub, rocks, logs back into your bucket. I'm not sure, but it looks like the part going back (on the jaw) might get in the way of the stuff you are grabbing? Second is that if you do get stuff inside those jaws, like shrubs, they might stay there. It looks like it may be hard to dislodge the stuff once it's behind that sharp corner.
Please don't take offense but this is my observation....I know how hard it is to hear about what you are making ... believe me. I think you need to use a more conventional shape jaw like the curved ones or angled ones that don't go towards the back so much.