Joseph,
I am aware of the shulchan aruch. I am talking about the people whose behavior you are trying to correct. They are also aware of the shulchan aruch and its halachos. Your question was about increasing the level of tzniyus.
“If someone doesn’t keep kosher I’d expect them to not keep tznius.”
Is the reverse also true (someone who DOES keep kosher also follows the halachos of tznius)? In fact, you yourself have posted numerous times, as have others, it is often those who adhere to the highest levels of kashrus when it comes to their food, are the ones who need the greatest amount of chizuk when it comes to their manner of dress. So, while it is true halacha is clear where it stands as far as the food one should eat and the manner a person should dress, you (meaning me, you and the community Rav) can not legislate that people follow the shulchan aruch, nor increase their level of yiras shamayim.
This is, as stated above, for the most part not an awareness issue, but a yiras shamayim issue with chizuk needed by many to overcome this particular yetzer hara.