I've got a single ceiling box/light fixture in my dining room which is wired up with BX. I'd like to have a second box about five feet from it. I'd run a new piece of BX nutted between the two boxes and then nut the wires to each fixture accordingly. The run is parallel to the joists so don't need to drill through those.
Thing is, I can't seem to find an "old work" ceiling box for BX. Why would they have nice deep ones for NM cable, but not for BX? Lowes has these "shallow" boxes, but not quite sure how they work. Maybe they are designed to hang another box off of them? No way you'd get a wire nut in there.