I am not convinced that a utilitarian legal system is much different than the systems of modern Western societies. Most laws in such societies are passed on grounds of ethics, efficiency, or a combination of both. Many people assume that laws passed on ethical grounds are inefficient by utilitarian standards, but I don't think that's necessarily true.

Consider murder laws. These are typically justified with a moral argument: life is sacred. But when a person is killed by another, the cost is not just some abstract violation of moral principle--since it is people that hold morals, and since those morals (usually) specify that loss of life is a bad thing, murders impose costs on members of society. These costs could conceivably be measured ("How much would you be willing to pay to revive person X?") and should be part of any cost-benefit calculation aimed at judging the value of a "moral" law. Of course, murder laws yield obvious gains in economic efficiency as well (disincentive to kill->more security->more commerce, peace of mind etc.), but I am considering only the moral side of things to make my point.

Why don't we have compulsory clinical trials? I wager they wouldn't pass a proper cost-benefit analysis.