Knowing what you currently know - If you were looking at budget carbon fiber tripods and had pretty much decided on the Hakuba HG-503MX but then you realized that it has no hook for a rock bag would that be a dealbreaker? I think I like everything else about it and I think for the most part I mostly wouldn't use it but I wonder if I would ever miss it. I supposed I could drill a hole in the bottom of the center column and install my own hook.
As advised above, be creative, but I would add: ALWAYS hang some weight to it. I owned one of the earlier carbon fiber tripods by Manfrotto. I always hung my backpack from it at the top. The first time I did it was a nerve wrecking experience because its thin legs all flexed, but then it gave me wonderful sharp images like the one below: