The practice of hanging "stuff" off the strap seems to surface in early photos (Tobin, Medina, etc), but as TC has pointed out, those guys were all from the southwest.
The earliest mention I've found of this practice is in Sage: Scenes in the Rocky Mountains was published in 1846 and discusses his observations from around 1840-1844. Ruxton mentions it a few years later. Both men spent time in the southwest, and I can't help but wonder if these descriptions didn't influence, more than describe, practices there. It really looks to me like it was a fad--an expectation that "this" was how a southwest mountaineer would carry his gear--and that it may have been started (or at least popularized) by Sage.
(In a similar manner, the mention of "Green River knives" in Ruxton seems to have lead to an expectation on the part of readers that "mountaineers carried Green River knives." Ruxton mentioned them first, then everyone tagged on when his newspaper series became popular. There have been more than a few people over the years who've suggested that Ruxton was trying to out-tall-tale the other writers of his day.)
And keep in mind that those bags are shown in photos. They are late bags, quite possibly the last bags these guys had. Absent other evidence, they shouldn't be taken as representative of earlier bags, or of bags used in other areas of the trans-Missouri west.
Some examples from other times/areas:
In Edward Warren, in what is considered by many to be a description of his own outfit of 1833, Stewart wrote, ". . . a butcher knife was in my belt, and an awl was attached to my pouch, which, with a large transparent horn of powder, and a wooden measure hanging to it, completed my outfit." An awl, a powder horn, a measure, and that was it for strap-hangers.
If you look at the few early illustrations, Miller and Bodmer don't seem to show a bunch of "stuff" hanging off the bag. In 1851-52, Kurz drew what is one of the earliest and best illustrations we have of a pouch used on the upper Missouri. Aside from a horn, and what appears to be a powder measure, there's no "stuff" hanging off the strap. (In the late '80s, I was told that Denig's pouch was still extant; has it ever surfaced in a museum or auction?)
A bag collected in the late 1850s that is believed to have belonged to Logan Fontenelle, has nothing but straps for a horn, with no sign of anything else having ever been attached to the strap.
A bag that can be dated to at least 1861, from Taos, has only a powder horn and a WBO striker attached to the strap. Everything else is either in the bag, or curiously, attached to the rifle (powder measure and pick were tied to the rear of the triggerguard).
Jim