Actually was thinking about this earlier today
I recall many years ago reading a period account describing a fight between the narrator, a frontiersman, and an Indian, in which the two eventually ended up wrestling each other for control of a knife that hung off the shooting pouch belonging to one of the participants. The person who posted the account pointed out that the fight would have taken place in the early 1790s, and thus was the earliest description of a knife mounted on a pouch. Unfortunately, I cannot recall the details or whose narrative it was, so you might want to treat that as hearsay. Might be worth tracking down, though.
Audubon's famous description of loading a flintlock, c.1810, sounds like it involved a knife attached to the shooting bag:
"… He blows through his rifle to ascertain that it is clear, examines his flint, and thrusts a feather into the touch-hole.
To a leathern bag swung at his side is attached a powder-horn; his sheath-knife is there also; below hangs a narrow strip of homespun linen. He takes from his bag a bullet, pulls with his teeth the wooden stopper from his powder-horn, lays the ball in one hand, and with the other pours the powder upon it until it is just overtopped. Raising the horn to his mouth, he again closes it with the stopper, and restores it to its place. He introduces the powder into the tube; springs the box of his gun, greases the "patch" over with some melted tallow, or damps it; then places it on the honey-combed muzzle of his piece. The bullet is placed on the patch over the bore, and pressed with the handle of the knife, which now trims the edge of the linen. The elastic hickory rod, held with both hands, smoothly pushes the ball to its bed; once, twice, thrice has it rebounded. The rifle leaps as it were into the hunters arms, the feather is drawn from the touch-hole, the powder fills the pan, which is closed. “Now I’m ready,” cries the woodsman….
I think you could make a decent case for an attached knife sheath for at least the last decade of the 18th century, and I personally wouldn't consider it out of line if you wanted to include it as a speculative detail for a kit a decade or so earlier...
Having said that, a couple observations. First, I'd imagine that any knife attached to a pouch would be a big butcher type knife, not a small fixed-blade utility or "patch knife". There isn't much evidence for handmade or blacksmith-made knives being at all common along the frontier prior to 1800, whereas it appears that virtually everyone used a trade knife of some sort. I have yet to run across an example of a trade knife with a
fixed blade much under five inches (if anyone knows of such a thing, I'd love to hear about it). The need for small blades seems to have been filled by various grades of folding knives, which need no sheath.
Second, a sheath attached to a pouch means that the knife is only handy if you are wearing the pouch at the time. That isn't a problem if you are out in the woods, but if you are working around the farm (i.e., most of your waking hours as a frontier farmer) you aren't going to be wearing the pouch all the time, which means that your knife is inaccessible if you need it for a quick chore. I use a utility knife all the time at work, and while it is supposed to be carried in my tool pouch, in practice I tend to leave the tool pouch in one place (cause it is awkward to carry all the time) and and either slip the razor knife into my pocket or just use my Opinel rather than go find my tool pouch when I need to cut something. I suspect that the knife most used (possibly that folding knife!) would inevitably end up carried on one's person, and any knife on the pouch would be either reserved for specific uses (weapon or dedicated hunting knife) or as a backup to one's favorite knife. In other words, while an all-in-one rig makes sense for us today, allowing us to don or shed the 18th century in a single movement, it seems rather impractical for an actual 18th century homesteader, so I suspect that the practice was not common.
