I don't know if there is a law in Islam that allows enforcement of women (an men) to be covered at legal proportions.
I know there is a hadith that talks about muslim women at the end of time that will dress while being undressed. The prophet then sais to curse them, since they are cursed by god. (so it seems like he didn't state a physical/legal punishment for that, but rather a social one). This is the point where the answer of @Ali786 becomes relevant: if people reject them, the society perceives them as disturbing cursed elements, chances of the behavior getting expanded are greatly reduced (as long as people will reject them)
Not long ago it was against law in Europe/US to be dressed shorter than a certain Cm count from the knees. There were laws for molestation, and it was clearly visible that the society didn't want undressed people. Their solutions were maybe too radical, and their beliefs too week, since Edward Bernays made nudity fashionable through Marilyn Monroe and some other stars, and people didn't know why they disliked nudity, so decency fastly disappeared from that part of the world (also thanks to the main stream media).
So to answer you even if I don't know about legal rules, the society should be the one who chooses laws that rule it, in a society that has a majority of muslims, the law should democratically, fairly, by revolution etc... inevitably fall into muslim rules.
The good point in islam is that you cannot add rules to override islamic rules or even complete them : if something has no clear evidence of being ruled by islam, you cannot say that islam didn't think about that point and invent something new :
ex: women getting more and more naked because of TV and media influence, so => we should stop by by adding a law that punished women without hijab
We know the book is from God, and God knows the future, so if something is not stated, it's not because God didn't know, but because the answer is elsewhere.
To get back to the Hijab subject, if the prophet didn't state a punishment for not wearing hijab, he stated that people should curse these women, which is a drastically better and more durable solution. And since we are here only for the exam, and each person will get rewarded for what he've done and the consequences of his acts (even thousands of years after his death), each person should follow the rules to be able to justify himself the day he will face God.
p.s: cursing is a behavior that is quite incompatible with islam, there is here a strong subtility that you cannot understand from where you seem to be. Anyway, keep in mind that cursing a muslim is always a kind cursing to remind him he is in a wrong way, it's not just to insult and feel better.