A few years ago I purchased an OMM rucksack chest pack wich just hooked to my shoulder pads.
I think you refer to the OMM Trio, a 4l pouch fixed on the backpack straps with two additional hooks to attach round the back to whatever points you could find. I had one and did a brief review of it on the site, I used it to carry a DSLR with zoom lens.
A few years ago I abandoned the whole idea of a big DSLR and the Trio became redundant, instead I carry a super-compact camera in my hipbelt pocket. I might consider a smaller pouch for a slightly larger camera if only there was a way to secure it to almost
any backpack shoulder straps in a way that would hold it very firmly. Most add-on pouches jiggle about to some extent which would drive me nuts.
Tread Lite looks interesting.