Working on rewiring a dozen 1910 gas/electric sconces to remediate some old K&T wiring, I'm up in central IL. When I pulled the escutcheon plate on the fixture I was greeted by a wonderful combination of the fixture hanging off the gas pipe and no electrical box. Upside it does have a UL rated insulating shut so that's good I guess lol. The gas lines for these were disconnected in the 1950's at the point where the line goes into the basement ceiling. Both myself and a plumber were able to trace the only gas lines still connected and confirm there is no connection to the old light lines.

Trying to piece together some strategy before I go cutting holes in the wall. Initial idea is to enlarge the ~1" hole in the plaster to fit a 4" old work round box and use the center hole to pass the gas line used for support. Then I could pull the old K&T wires in and cap them off and run new NM cable up the wall.

Not sure how code compliant having the unused gas line pass through the box for support is though? If I get lucky and both sides of the K&T wiring come up from the basement can I just cut them there, pull the live ends out of the ceiling to a J box until the last of the circuit is dead then shove them back into the ceiling? Or do I need to pull out enough of the lath/plaster to get to the last knob and terminate there? Any good resources on dealing with old K&T circuits, or is this pretty jurisdiction specific?

If I get really lucky and the gas line is only supported at the ends and I could yank it out, could a old work box in lath/plaster support a sconce hanging from a nipple in the box? Also then I would have a stub line in the basement ceiling that was capped off, is it permissible to have a capped line in a cavity or does it need to be in the room?