Ah, but this is a room of ladies, rather than a room for ladies. If you require an actress, for example, or a female voiceover artist, or a woman to be photographed for a magazine advertisement, you might go to the room of ladies in order to meet someone who can help. Non-ladies who require a lady are permitted to enter. 'Only Ladies' would be closer, but it's still not entirely clear.

It's possible that I'm thinking too much about this. I am unlikely to ever actually be called upon to label a room full of ladies.