Well sounds a bit like the liberal theory of a social contract.
If your question is "could liberal democracy replace laws with a social contract that is totally self enforced?": I would say no but they will keep trying that's what biopower is.
If your question is "could a social contract be used in anarchy": I would say at the risk of sounding prefigurative... NO its hard for me to imagine some kind of written code, self made or otherwise, not becoming a form of social control reminiscent of state power.
Could informal agreements play a role in improving how people treat each other? yeah. maybe the idea of a contract just smacks to much of John Locke.