Possibly in a different category than you are looking for, but if you make use of 2 on the belt and 2 on the shoulder straps you can get an awful lot of individually small but awkward thing packed outside.
These are pretty sweet (from Elemental Horizons):
I got these in large for my Kalais Pack but they fit on all my other packs nicely. They have two sizes, and the large is about the same size I had built into my old ULA P2. They have the straps that go around the back as well as the clips that attach into the webbing strap on the belt. As good as they get. You could get a down jacket in one of them easily, or plenty of other stuff.
Zimmerbuilt makes some other nice attachments of all types. I particularly like the padded shoulder attachment, but he has belt pockets (and other types) as well:
Perfect for my tiny digital camera. Until I found that one I had about half a dozen versions of horribly ugly padded containers I made out of bubble wrap and tyvek tape. :-o
If you a going for shear volume because your pack is too small instead of just wanting easy access you could consider getting something custom built from zimmerbuilt, he is good, quick and reasonably priced.