Not sure where this "under the law" stuff is coming from, as the two most obvious scriptures applying to Christian women dressing modestly occur in 1 Timothy and 1 Peter, and neither Paul nor Peter were legalists.
We can go back and forth with this all day long, but these discussions always come down to men and women having lust issues to be dealt with in their walk with Christ, and, regardless of gender, we need to take responsibility for our own actions, whether it be the way we allow our thought lives to progress or the way we dress.
If God puts it on a woman's heart that the way she is dressing may be an issue and she may be causing someone to stumble, who are we to tell her she is wrong? I learned this the hard way as a very young Christian. I was driving by and saw an elderly pastor's wife dressed somberly, with her long hair pinned up, no makeup, etc., and in my newly found self-righteousness I thought, "Lord, I wish she knew she did not have to dress that way." BUT... God took me to the woodshed over that and showed me that:
1. She was the woman who had prayed for the miraculous restoration of my healing;
2. That I had absolutely NO right to judge her for what she did or did not choose to wear because her heart and BODY were totally HIS;
and
3. When someone is led to DO or NOT do a thing out of a pure desire to honor/obey/worship HIM, He honors it and I need to keep my mouth shut (2 Samuel 6), as there are plenty of things in my OWN life that need my attention.
ON THE OTHERHAND, we need to remember that we are not the Holy Spirit and should never take it upon ourselves to do the Holy Spirit's job in "cleaning someone up". Our first concern should be the person's spiritual condition, which is not something that is outwardly ascertained. If we keep the main thing the main thing, and lovingly encourage a person's walk with the Lord (whether male or female), prayer life and understanding of scripture, I assure you that God is able to guide them into how they should be dressing.
I have participated in dance (mainly ballet) and gymnastics all of my life, and some of the clothing I wore in those activities never bothered me until the past couple of years....until I posted a photograph of myself in a dance costume that showed two thirds of one leg and my abs (I posted it in response to an innocent conversation about P90X workouts where I had concerns that the workouts were beginning to make my abs look too manly), and received an inappropriate message from a married man on my friend's list. Yes, what he did was wrong, but since I can't say with absolute certainty that I was dressed appropriately, I won't be doing that again. He has to take responsibility for his thoughts and words, but I have to realize that I may not have been completely innocent either, whether intentionally or not.
We are not children. We understand the concept of lust. We understand that men (and some women) are visually stimulated and they fight a tremendous battle. If we are truly trying to live out the concept of considering others above ourselves (Philippians 2) is it really too much to ask for us to put some clothes on? Thoughtfulness and selflessness does not equal legalism, but fingerpointing sure does.