Vitamin C can also be used as a developer. I did a quick google search and came up with this recipe:

8 oz of water
8 vitamin C tablets (1000mg each)
5 teaspoons of washing soda
Develop for 30 minutes, agitating every 30 seconds.

If you had citrus fruit on your island, and you could extract the vitamin C. However, the washing soda is needed as an activator. So, don't know about where one would run across that naturally.
An interesting article called Coffee, Tea or Vitamin C, Kitchen Chemistry in the Darkroom can be found here.