I have never tried salt. The washing soda is available in Walmart stores. If you cannot find it, you can take regular baking soda and bake it in a hot oven for about half an hour to make your own. Google will tell you the exact temperature and time.
The long rod supported by rubber rings or similar will work great. What I used on one of my cleanings was some of the spiral wrap plastic wire sleeving. If you stretch it out you can leave an exposed band about as wide as the insulating band all down the length of the rod. A little electrical tape held mine in place.
I cannot think of a good thing to compare the smut to. The closest thing is the scuzz that builds up inside of cast iron sewer pipes. The particles are very fine and seem to get into your pores easily. You can rinse most of it off with a garden hose. A pressure washer would be better. For frame rails and such you could also try the type of brush used to clean out clothes dryer ducts. Only a bit of gentle brushing is needed, you will not need to wire brush the surface unless you want to. However, you will want to really clean any surfaces you plan on top coating. Any lose particles on the surface will make it hard for paint to stick firmly.
As long as the surface you want to clean is below the water level, the electrolysis will clean it. Rather than dip all of a fender or similar, you could take your time rolling different areas of a big part through the bath. As for a whole body tub? I would not try something that ambitious. However, that may just be me. If I were going to do an entire tub or frame, I would take the parts to a chemical strip and coating place. Down here I took the bonnet from our GT6 to a place called Carolina Chemstrip. They stripped the bonnet and gave it a dunk in the phosphoric acid vat to remove the rust and leave an iron phosphate surface for painting. It was relatively affordable... but that was 25 years ago.