Three "tricks" come to mind.
First, try to go during "off peak" times. If there is an intermission, there will be a line. But the line will be much shorter before or during the performance.
Second, women often go to the ladies' room together. So if your wife is comfortable with it, you both get in line and quietly chat with each other. That looks perfectly normal and another woman is very unlikely to interrupt, and, if she somehow does, your wife can field the conversation.
Third, if it does happen and you are alone, return her eye contact, smile (always!) and respond in a soft voice. Most women are pretty good at lip reading, even though they don't realize it, and she'll fill in the blanks automatically. You can almost move your lips without actually vocalizing and it is likely to work, unless she has suddenly come up with some convoluted question like "Can you tell me how to get to the airport from here?"
Do you have experience with using the ladies' room itself? Particularly under crowded conditions? Do what you need to as quickly as you can (but make sure your dress isn't caught in your pantyhose!), always use some toilet paper, always flush, definitely do not put the seat up!
. Always wash your hands (I've noticed that women tend to wash with their fingers together while men tend to interlace fingers when washing), and always use a mirror (sometimes there are ones located away from the sinks so that you don't tie up traffic) to check your hair and makeup.
Have a great time!
Hugs,
Persephone.