More to tznuis than meets the eye

Last night, Chavie and I got into it again.  The topic--tznius.  Chavie mentioned that her daughters don't dress up for Purim after Bas Mitzvah.  My response--"what's the fun in that?"  (I love dressing up on Purim.)
Chavie: "It's not tznius."
Me: "It's Purim."
Chavie: "You NEVER take a break from tznius!"  Chavie then went on to go into how Torah is for men, and tznius is for women.  I then countered with the fact that most of the specific halachot on tznuis came from the Mishnah Berurah--a fairly recent source.  I also pulled out some fashion history, such as that at one time, no woman would have dreamed of showing her legs, but nearly all women showed their decolletage--yet the reverse is true today.  Chavie responded that the importance of tznius for women comes from the Torah.  Of course, she couldn't quote me chapter and verse.  Know why?  IT'S NOT THERE!!!!!!!!!!    The only reference she could come up with was that when the malachim visited Abraham, Sarah was in the tent--exactly where I'd be during a heatwave.  (For those who don't remember--Abraham was recovering from his circumcision, so G-d sent a heatwave to keep everyone away.  But Abraham wanted visitors, so G-d sent three men.)  I also pointed out the irony of discussing this during the only parsha in the Torah which mentions a dress code, Tetzaveh--which discusses the beautiful, brightly colored, bejeweled, golden garments of the Kohanim. 
Chavie:  "But of course they wore brightly colored clothing.  They were royalty!"
Me: "Aren't women supposed to be princesses?"
Of course they are.  But unlike Kate Middleton, we show our royal status by covering up and hiding away. 
Of course, I wanted to know the source for tznius being for women the way Torah is for men.  She mentioned that women have a yetzer hara not to be tzniusdik the way men have a yetzer hara not to learn.  (Since Builder spends a lot of spare time with a Gemara, that doesn't fly with me.)  So, I looked it up.  Big surprise, the sentiment was found nowhere in the Torah.  It wasn't in the Talmud.  It wasn't even in the Shulchan Aruch.  Apparently, the Vilna Gaon had written a letter to his mother (the following is excerpted from Congregation Toras Chaim's website):
When the Vilna Gaon set out on a journey (that he hoped would take him to Eretz Yisroel), he sent a letter of chizuk (strength) to his family, known as the Igeres HaGra. In it he warned them about the need to avoid anger, arguments, jealousy and similar bad midos. In particular, he stressed the gravity of the sin of lashon horah and other speech-related aveiros. Towards the end of the letter, he addresses his mother with the following words: אהובתי אמי, ידעתי שאינך צריכה למוסר שלי כי ידעתי כי צנועה את – “My dear mother, I know that you do not require my mussar, for I am aware that you are a tznua (very modest person).” Although the mussar (ethical and moral guidance) given in the letter concerned all types of negative traits, he was nevertheless convinced that his mother, who was an outstanding tznua, was above all negative traits and did not require guidance from him to overcome anger, lashon horah, and the like. He was convinced, that just as being steeped in Torah enables a man to combat his “lower self,” so too, being steeped in tznius enables a woman to be victorious in the same way. He therefore knew that his mother, who was an exceptional tznua, would surely overcome whatever test she would encounter.
Now, the last time I checked, the Vilna Gaon lived in the 18th century.  Back then, even prostitutes dressed somewhat modestly.  Tznius, then, had to refer to more than just dress.  It is an attitude, a lifestyle.  And, yes, it is important.  But somehow, I think there is more to it than collars, wigs, skirts and stockings.