Joseph: I will disagree with you on this one. When it comes to Tznius, the idea of having a man notice and rebuke a lack of tznius on the part of a female dressed inappropriately, is in itself lacking in tznius. There are other ways around it, like mentioning to a female who you have in common with the “tznius offender” to sensitively mention it.
You have to realize that in this topic, women are very sensitive and it simply isn’t effective to “give tochacha”. As I have mentioned previously, I work with bais yaakov girls, and I can tell you the situation is far from simple. It cannot be a open and shut case that tznius is like every other mitzva which you can go over to anyone to give tochacha. It requires a lot of sensitivity and know-how for it to be effective. Girls and women just do not appreciate being told that they dress inappropriately and certainly not from men. It is a tough sitaution, but sadly true.
Additionally, I have an issue with men going into specifics about women’s clothing. There is a widely -studied book (sefer ) on all tznius areas possible which has become quite controvertial, partly because it was written by a man, and was very specific. People have been very turned off by the whole idea. There are b”h enough women out there who can tackle these subjects in a sensitive and refined manner.
In summary, while you may be in the “right” (and i’m not sure you are) that when you see an area that needs tochacha it should not matter what mitzva it is (shabbos, tznius or otherwise), it is more important to be effective than to be right.