Doc, my take on 'picketing the horses' is that those two guys in front are definitely wearing breeches (buckskin or cloth, unsure). They're also stockingless, sans leggings, and barefoot. I doubt you'd take this stuff off before diving into setting up camp after a long day of travel, however they do have a tent up in the back already. Who knows if they were just laz-ing around camp or if that's the way they normally went about. The guy with the wolf-ear cap is in a pair of pantaloons and the guy driving the picket may be wearing drop-front trousers or pantaloons. There's also folks in the sketch in typical plains indian garb. That sketch really tells me that there was no strict uniform, but a variety of options.
I agree that some get a little too caught up in the details, and that Miller may have had a tendency to display people as AFU'd (all fringed up) for romanticism. Store bought clothing appears to have been rather common in the rockies from trade records, but it may have been to everyday to be deemed paintable. Still there are interesting details like cruppers, saw-buck packsaddles, and some clothing issues that can be taken from these. Like anything else, pulling conclusions from just one source is always sketchy.
Sean