Gee, where's Knob & Tube when you want it? Individual wires would be nearly invisible
Painting galvanized metal is completely dependent on how long the stuff has been exposed to the sunshine and weather, and just how much zinc was put on.
The weather removes the coat of light oil on the metal, and paint doesn't always like to stick to zinc. Hence the vinegar trick; as a weak acit it removed the oil AND added some 'tooth' to the zinc for the paint to grab onto.
MC, even the aluminun jacket form, has plenty of oil on it. Once the oil evaporates, it doesn't take long for the jacket to lose its' shine.
Exposed? I'd just make sure to run it real neat .... wipe it with a rag soaked in paint thinner .... then take advantage of the grooved surface to wrap some yarn around it. I think the yarn - especially if it matched the color of the ceiling - would really break up the outline of the jacket. Any other metal - straps, boxed, fittings - I'd just paint a satin black ('flat' looks dusty or dirty).