Threaded rods are a popular choice for both nuts and bridges on cigar box guitars. If you are going to use a threaded rod as your nut, and you are concerned about getting intonation just right (whether you have a fully fretted neck or just have fret locations marked), always remember that the scale length is based on the vibrating length of the string.
This means it starts being measured from the point where the string leaves the nut and bridge. Because threaded rods are round, you might have to adjust their location slightly to make sure the string is losing contact with them at the same point it would with a standard rectangular/angled nut or bridge.