Years back, in an old structure that was an American Legion Hall & Offices, the second floor area was all K&T. The surface twist style switches had wooden molding with grooves routed for the conductors. I fondly remember hunting for a fuse box, only to have one of the senior members say 'sonny, it's located in the baseboard right here!' He was 100% right on, two circuits, four fuses block, custom 'box' lined with 1/2" asbestos (the real thing) board with the baseboard neatly mitered to hide it.
In Europe this was certainly common practice, even though in finished living areas usually twisted lamp cord was used rather than individual conductors. The latter was commonly used in unfinished and possibly damp areas. The Austrian Parliament is still wired like this to some extent, I saw pictures in a newspaper report about the state of the building (of course this wiring method is confined to the non-public areas, but it was also common in residential basements and the like well into the 1920s). In Germany, exposed wiring on knobs was apparently outlawed in 1910, in other countries it remained in use much longer. The wooden "conduit" trim was commonly used in the UK and Ireland and can still be found in some older buildings (I think I posted pictures of it in a Belfast hostel here a few years ago).
There were receptacles and switches made for just what you describe- to be set atop surface-run wires. Often these wires would run just under the final plaster coat. maybe an inch apart. Another trick was to run them behind door moldingas and baseboards.
This was strange looking because there were 2 wires, insulated I suppose, running an inch or so apart with a porcelain insulator holding them apart and a round switch below that. I am really mad at myself that I didn't get a screen shot.