I don't use salt, but I take kelp tablets every day - except just now when I have to go and buy another tub, but usually - and I have to avoid grain, all carbohydrates really, but grains in particular as I find it is why I put on weight so easily.
When I cut out grain my digestion improved, my joints got better, and my waist measurement reduced 12 inches in a few months. My dietician had three fits as she regards grain as an important part of the Human diet - even though it never existed until about five thousand years ago, less than an eye blink in Human evolution.
I do wonder if there could be a diet element in Dupuytrens, as there does appear to be a wide range of effect, even with the genetic plus RSI elements. If you are really unlucky the finger is fully bent down, whilst others just have a strange hole in the palm and a slighly pulled tendon.