Great idea. Buck and I used to camp all the time. One time, after making a particularly "goopy" dish in a piece of our cast-iron, I discovered I was without a scrub pad to clean it.
I improvised and use the eggshells (crushed up) from part of the meal to scrub out the pan. They worked great and I've been doing that ever since, even at home.
Necessity is a...mother. Of invention, that is.