When the chapel where I generally attend Mass decided to investigate inviting the SSPX to the chapel, the question of acceptable clothing was asked of the SSPX. Since the chapel was first established in the 1970s, the priests who came to the chapel asked the ushers to make sure people who came into the church were properly dressed; at the very least, they were to explain the dress code to make sure visitors knew that they would not be welcome dressed as they were for a second visit. The SSPX said that the laity should not talk to people about their dress at Mass.
As an example of what I am saying, if a woman were to come wearing slacks, the requirement of dresses/skirts extending to below the knee would be explained. Head covering was provided. She would be asked not to approach the communion rail improperly dressed. If a man came in wearing jeans, he would be advised of the "no jeans" policy and also asked not to approach the communion rail. People in shorts/mini-skirts were not admitted to the chapel.
The SSPX explained that the priest that came to the chapel on Sundays and holy days would determine the appropriate dress in the chapel and that ushers WOULD NOT address the dress of any visitor. Period. If people were inappropriately dressed, the ushers could talk to the priest and the priest would address the issue himself. Of course, the priest is in the confessional for an hour before Mass and goes directly from the confessional to the sacristy to dress for Mass so the issue cannot usually be addressed prior to Mass.
However, based on what has been completely acceptable at our chapel as defined by the priest not addressing the issue:
Woman's skirts can now be up to a couple of inches above the knee.
Woman may wear pants
Woman need not wear anything on their heads
Jeans on men are ok as long as they're the good jeans
Earrings (in one or both ears) on men are acceptable
Hoodie's are acceptable outer-wear (though I admit he wears slacks and a tie)