The best way to buffer a grab is to press both shield and attack individually, at the same time. I don't even use Z anymore, due to the poor macro system of Brawl.
Also, your post in the Marth boards was fine, your title was just deceptive, and made me think you were talking about something that we were JUST talking about in another thread. That's why at first I locked it, then I saw the OP was talking about something slightly different, and I unlocked it and changed the title.
However, what you just said is false. If you glide toss into my shield, I will have frame advantage and be able to grab Diddy first. Otherwise, Diddy would be a broken character, ROFL.