Talking about natural stain removers made me go re-read the washing tips: "Diaper Stains – Rinse soiled diapers promptly in the toilet to minimize stains. Biodegradable paper liners or Stay Dry liners will also help. Should you have a tough stain, soak it in a concentrated solution of Ivory Snow or Dreft gently rubbing occasionally until the stain has lifted. Repeat if necessary."
You mention under line-drying that sunshine is a natural way to bleach your diapers, but how about giving another plug here? It works miracles, esp on BF poo, and no worries about whether or not it's safe!