ISTM, the point to be made is that fashion and modesty are both culturally-determined. The Bible teaches modesty, but does not teach fashion.
Now, if a particular church wants to define itself as a separate culture or sub-culture, with its own fashions different from the surrounding culture (like the Amish, for instance) there probably isn't anything wrong with that per se. A Baptist church that requires women to wear dresses and not pants would simply be making a statement that they choose to be culturally distinct.
I think the question should not be whether or not it is appropriate for women to wear slacks, but what things should distinguish the church from the rest of the world, and why? The amount of cultural difference the Amish practice is a huge barrier to evangelism. New people are not easily incorporated into their culture. As a result, they don't often preach the gospel to outsiders.
I think RP's approach is a good one - much better than having rules and preaching them from the pulpit. There is a young lady who has been attending my church for almost 2 years. I don't think she dresses appropriately for church. She often wears halter or strapless tops in summer, and often shows some skin between her top and her skirt or pants. But she is a new Christian and loves the Lord very much. It's probably time for someone to speak to her about the way she dresses for church, but I am not going to do it myself, because this should not be communicated to her in a way that will seem non-accepting of her, but rather by someone who is helping her grow spiritually. I don't have that kind of relationship with her. I plan to speak to her aunt and suggest it may be time to talk to her about this. But before now, I really didn't think it was appropriate.
OTOH, I have no problem speaking to the younger women with whom I have a close or mentoring type of relationship. I have been known to take a younger woman aside and tell her next time she wears that skirt, she should be sure to wear a slip, or something similar, but my first concern is not to turn away a new believer.
DOIA, I think this would be something to discuss first with your wife, and then maybe with your pastor. Do you both feel like you want to be part of a church that distinguishes itself culturally this much? Do this and other cultural distinctives adopted by your church help or hinder evangelism in your community?
I have seen churches that observed certain traditions about dress grow and be welcoming, and others not. For example, in a community where I used to live, there was a large and fast-growing Eastern Orthodox church. When I was invited to visit, I think my friend told me I should wear a below-the-knee skirt and be sure my shoulders were covered. I can't remember exactly how that came up. I may have asked her first whether I should wear some sort of head covering there. Anyway, it was simple respect for that church's culture. She told me in a way that indicated her concern that I feel comfortable there, and respect for my concern that I not draw attention to myself as someone who was culturally inconsiderate.
Fashion is a matter of culture, and it may be good or bad to make a distinctive cultural statement as a church. How you do it and why are very important. Also, it is important that it be communicated in a way that does not lead to misunderstandings about the reasons. it should enhance evangelism and fellowship, not put up barriers to evangelism and fellowship.
Just my $.02.
