Theoretically, when there's only one Beis Din in town, it can rule any decrees and customs and exert power to maintain it. So this type of Mishmeres would be doable in a small Shtetl.
The problem starts with two or more Batey Din in the same town and any Jew can deny his subjection to any of them.
I live in Jerusalem and I visit Meah Shearim occasionally. Let's say I wear a red shirt and have no Tzitzis, hat, or coat. The Mishmeres guy stop me and read me the decision of the Badat"z of the "Ha'Edah" regarding Tznius measures preparing to beat me up. Then I say: "Wait a minute, "מי שמך לאיש שר ושופט עלינו" (Ex 2.14)? I belong to a different community and you have no jurisdiction over me!". So they smile ashamed and humiliated and leave me alone.
Your question is based on a presumption that some Beis Din's judgments are Halachicly absolute and universal, which is definitely false. Men and women in different communities dress and behave differently, whatever is considered modest in one community is unacceptable in others and there's no central authority accepted universally by everyone.
Regarding the power of intimidation or corporal punishment, the Halachah did not change, and when a court appoints its messengers, they have its full authority to exercise physical punishment, similarly to Biblical lashes.