A small French study of 20 patients with painful RLS trialled 950mg of a mushroom supplement called Ganoderna Lucida. 13 reported positive results.It's a tiny study and they didn't include a placebo group.
But if you experience pain as part of your RLS, it may be a useful addition. The study reported only mild adverse effects in 2 patients. A headache for one day and mild diarrhoea in another patient for a short time.