when I was in the boyscouts, we learned to coat the bottom of pans with soap, but we didn't use liquid soap, we used bars of soap and they did the trick just fine. You could cook over an open flame and the pots would be no worse for wear.
and the bar soap doesn't make a mess, I wouldn't hesitate to use it indoors as far as a mess, but I'm not a chemist and have no idea if heating soap puts off some kind of toxic fumes.