Jack, I expect much of the argument will hinge on the theological position taken on the relationship between the Old and New Testament, as you hinted above. “Dispensationalists” are more likely to agree that laws for Old Testament Israel don’t apply today. For those who appeal to the Old Testament, you can point out that for the majority of the commands in the Law of Moses (e.g. payment of the tithe), there is no penalty prescribed for disobedience. This at least leaves open the possibility that ancient Israel was to employ non-coercive means in enforcing many of these “laws.” That won’t get you to full libertarianism, but it’s a start.