This is an interesting question to think about. Let me try to add my two cents to the discussion:

I think for me, the real question underlying the morality of a law is whether the law is a good exercise of power written to benefit the people living under it. So, for me, a law that disproportionately affects women, such as wearing hijab, should be a law made by a body that includes significant female representation. If the women in the country are making the laws to wear hijab, it seems less problematic for me because they are making laws for themselves.

But if a governing body of men are making laws for a group that is not in power, then I see it as an abuse of power. Of course, those men could argue that it is for the benefit of women, but, without female representation, we don't know if that's what they feel would be beneficial.

In that vein, the morality of laws governing female dress in the United States would be improved with more female representation formally agreeing on those laws.

That being said, as mentioned previously, wearing hijab is a much higher commitment than wearing a t-shirt/sports bra/bikini top and (I'm sure) affects daily life on a grander scale. Obviously, women who elect to wear hijab are making that commitment for themselves, so it isn't problematic. However, women who are told what to do by men who don't impose those same requirements on themselves are not being treated justly or morally, in my opinion. I don't know if it exactly answers what you are getting at, but I think the scale of representation by a group and how much a law impacts daily life for that group affects the morality of the law.

(Notice that I did not mention whether I think it is moral to require hijab by law. I honestly don't know much about female representation in places where it is a requirement, but my argument is basically that the morality of that law depends on the circumstances surrounding it, not on the law itself.) (Also, in an ideal world, it would be best to have non-believer citizenry represented as well, although obviously that would be difficult.)