Instead of salt-firing ceramic to glaze it, could you glaze it by producing a sodium-containing plasma on or just above its surface? For example, you could blow powdered sodium chloride into a plasma arc to vaporize it, or you could generate the arc from consumable wire electrodes embedded in a solid rod of sodium chloride, perhaps mixed with fillers to add gas volume — for example, sodium bicarbonate (which would also serve as a chlorine-free source of sodium, but might produce carbon), sodium nitrate (which wouldn’t produce carbon but would produce nitrogen oxides that are worse than chlorine), boric acid, or sodium borate (borax).