For me as a Reconstructionist Jew, the values that I focus on when thinking about how I dress are; comfort, respect (both for my body and for the community), and necessity. Clothing ought to be comfortable when you wear it and should also show respect for the body and the customs of the community I am in. Finally, clothing should also be practical. As an active outdoors person I am struck by the number of people I come across who are dress inadequately to be outside. It isn’t safe, and doesn’t represent the best values of our tradition. The same is true in my work. If clothing looks good, but doesn’t help me be a better rabbi then I probably don’t need to wear it. Additionally, I personally find that wearing clothing with lots of labels takes away from the individual person and so I prefer clothes without identifying markers that are visibly printed.
Certainly there are other values at play for me; tradition and tzniyut. Reconstructionists believe that the “past has a vote, but not a veto.” That means the customs of our ancestors should always be considered and given weight within a conversation of values. Although many liberal Jews wears clothing mixed with wool and linen, I personally don’t wear shatnez. Although there is no particular reason, I understand that an ancient custom has importance and I appreciate being able to provide the link in that chain of transmission.
Modesty is a more complex issue and I believe has personal tones to it that need to be carefully considered. Clothing should show respect for the body and care of the soul. But the kinds of clothes that meet those goals are different for different folks. I get frustrated when people complain or judge other folks because they don’t approve of how they dress. And this happens at the synagogue, particularly around young teens. I remember once receiving a note from a board member of a synagogue I worked at complaining that I wore sandals on the bima. And I recall thinking at that time, why was he looking at my feet during my devar torah! He couldn’t imagine why I made that choice, and I couldn’t imagine why he was concerned about it. I just think there are better ways to build community and show respect for our individual differences and preferences.
Certainly a Reconstructionist understanding of tzniyut is different than some more traditional approaches. In the end, however, I think the values that guide the conversation are the same. The beauty of Judaism is that we can all travel the same journey, end up in different places, and continue the conversation moving forward.
I hope I can do as much justice to explaining the concept of tziut as certain individuals have recently done to corrupt it. Tziut is a lot more than modest; it is the Jewish concept of self. Properly translated as concealment, it is a philosophy of how to "be there" and not at the same time. It's about putting your best face forward by reserving yourself at the same time. The way you dress, speak, act are all part of creating a public you as opposed you the private you. The point is to focus people on the person's human (spiritual/intellectual) qualities and not their animalistic (physical) qualities.
Judaism is intellect centered. It is not anti-physical, but rather sees the physical is seen as a vehicle to refine the spiritual essence. The opposite of tziut is pritzut, bursting out. To contrast the two, a tziut person will discuss the finer points of Torah or engage in some other intellectual dialogue whereas a parutz (one who engages in pritzut) will instead gossip or tell you things about themselves you would never want to know. A tziut person lives to uphold social order whereas a parutz looks to undo it.
The most obvious application of the principle is dress. A Jew is supposed to wear nice, clean clothes that do not direct a person to look at their physical attributes. Bright colors, outlandish costumes, and the revealing of skin can all fall under the category of un-tziut dress. These requirements apply to both men and women, but are usually discussed in terms of women because of simple sociological realities. Men are attracted in a more base way than women. For example, a man will usually be automatically attracted to a scantly-clad woman whereas any woman will tell you that there are parts of a man that are distinctly not attractive.
Tziut is actually very logical. You wouldn't share your credit card information with a stranger or details of your private life (well actually I have had random people spill their guts to me but I assure you it was quite awkward). Why would you share your body the same way? Isn't your body worth at least as much as your money?
I can see of course that I lack a certain street credit as a male that people see as dictating rules upon women. I recommend a book called A Return to Modesty by Wendy Shalit. She does an excellent job of presenting the concept from a women's non-religious perspective. She can do it more justice than I ever could.
To address the crux of your question, the only difference between the traditional Jewish and the Reform/Conservative/Reconstructist stance on tziut is whether the guidelines of tziut are governed by Jewish law and Jewish tradition or are they a function of what non-Jewish society deems appropriate. I don't think you will find anything particularly different written from the perspective of a Reform/Conservative/Reconstructist rabbi.
In your question, you place Conservative Judaism in the same category as Orthodoxy. Although Conservative Judaism is generally more traditional than Reform and Reconstructionist Judaism, and although it views halacha (Jewish law) more similarly to the way it is viewed within Orthodoxy than to the way it is viewed within the movements which you call “liberal,” the dress codes within Conservative communities tend to be more similar to the dress codes within the liberal movements than to the dress codes within the traditional movements. Having said that, Conservative Judaism, like Orthodox Judaism, values tz’niut (modest) and believes that the way we dress is a way of reflecting this value and the belief that we are all created b’tzelem Elohim (in God’s image).
Formally there is little or no discussion of tzniut, understood as a standard of dress, within the liberal branches of Judaism. There is a common concern that one should dress respectfully when attending services, though the definition of what constitutes respectful dress may vary widely. So the simple answer is that there is no one position on how one should dress within the liberal branches of Judaism.
Nonetheless, the concern for modesty in one's life is shared across the board. Tzniut, after all, is not merely about what one wears. One can dress modestly while acting quite the opposite. Tzniut, as understood by our sages, includes matters of dress, but also speech and behavior.
On these various issues there is a great deal of discussion among the non-Orthodox movements. Within the Reform Movement, Rabbi Eric Yoffe initiated a curriculum for teens and families several years ago, entitled Sacred Choices, which teaches sexual ethics, but addresses a broader range of issues including modesty. If one searches on the website of the Conservative Movement for the term “modesty”, a number of discussions come up, including a significant number of articles on the Koach site written by Conservative-affiliated college students. A search of the Reconstructionist Movement's website yields a curriculum which examines dress as an ethical issue.
An attitude of modesty affects more than one's style of dress. If immodest dress can be seen as flaunting one's self before others, so can boasting and gossiping, over-reaching in business and social relations, and the excessive acquisition of material goods. These are all ways in which we say that we are more important than anything else, including God. Behaving in all matters with tzniut reminds us that we, like all other people, are creatures of the Holy One of Creation.