If it were mine, I'd try a stiff scrub brush, a metal tooth dog comb, or a metal brush for cleaning paint brushes. You will find sizes from small to 6" long. Go slow to avoid pulling the fabric.
For similar conditions, I'd use painter's bibs in cotton, Carhartt work bibs, upland bird chaps (light weight), or upland bird pants. The harder the fabric the greater the noise. The Carhartt bibs would be water resistant, the chaps and upland pants are very water resistant. If your hunting from a blind and don't have to move around they will probably work. The chaps are easy to remove when your in the blind.