Cohen's insight that the deliverances of pure justice can be combined with other variables to yield "rules of regulation" or a vision of a good society is applicable to liberalism.
Take Rothbardian anarcho-capitalism as laid out in his Ethics of Liberty, for example. Anarchists have been rebuked with queries like, without government: "How will crime be deterred?" "How will road building be financed?" or "How will air pollution be managed?"
Some of these are less easily answered than others. For example, anarchists have argued that a "monopoly" government will, as per the economic analysis of legal monopolies, tend to degenerate by providing increasingly worse services at increasingly higher prices. But the monopoly of force is not some accidental bug of government, to be fixed with "protection agencies"; it is its absolutely essential feature. The executive branch of the state must be powerful enough to crush with ease any identifiable criminal subgroup within a community, yet itself be small enough to be amenable to control via the legislature and courts. In order to make the state so powerful within its jurisdiction, all potential competitors to it must be thoroughly outlawed and, if one happened to arise, hunted down.
The job of the state is to dissolve social bonds, to isolate and neutralize criminals. This acid is of course highly dangerous and has been used extremely poorly. History is littered with remains of intermediate institutions beloved by conservatives that were destroyed by the all-powerful state. Nevertheless, this hazardous substance cannot be done without.
Let me suggest, however sketchily, that crime and road building are decidedly local affairs. It proves at the most the usefulness of cities, not of empires or nation-states. Air pollution is not much of an issue, I think; the solution to it is simply better technology, so that higher production can be paired with lower pollution.
In any case, it is probably true that pure anarcho-capitalism is unattainable in practice.
But so what? Justice still demands it. We care for things other than justice, such as deterrence of crimes and punishment of criminals, production of certain special public goods like intercity roads, control of externalities such as through basic sanitation legislation, and suchlike. Combining all these still yields a libertarian laissez-faire economy with, however, a minimal government to take care of such matters.
Consider, for example, the problem of (local) taxes. Some people say they are "proud" to pay them. Anarchists shower them with contempt for this attitude. Why, if you are so proud, don't you pay more? You are proud; but why do you coerce others into paying, as well? Etc. But consider how Hume characterized government: "Thus bridges are built; harbors opened; ramparts raised; canals formed; fleets equipped; and armies disciplined everywhere, by the care of government, which, though composed of men subject to all human infirmities, becomes, by one of the finest and most subtle inventions imaginable, a composition, which is, in some measure, exempted from all these infirmities." (Treatise, 3.2.7) A person is proud that he has successfully cooperated with his fellow citizens to implement this "subtle invention," this uniquely important technology.
To argue against anarcho-capitalism, one must show not that it is somewhat impractical, for I grant that, but that it is unjust, and no one has ever done this successfully.
Anarcho-capitalism as it stands is unachievable, but if it were, it'd have to be put into practice as per the recommendation of justice.
I agree with Cohen that the teachings of justice stand or fall on their own, regardless of other considerations. I disagree with him regarding what is just: he prefers his egalitarian socialism; I, my Rothbardian anarchy. Again I agree that both are pipe dreams.
Far be it from me, however, to concede any other parity between these theories of justice. Perfectly just yet a little impractical anarcho-capitalism is very close to a slightly less just but fully workable libertarian minarchism; Cohen's vision is light years away from anything resembling a sane economic and social system.