LOUISE: I’ll be the lady-in-waiting. The one with the low-cut blue velvet Renaissance dress.
PARIS: Lady-in-waiting is not a political office.
LOUISE: No, but they get all the sex.
LOUISE: Watch a movie.
In period films, it is fairly common to have the lady-in-waiting be the “sexy” figure, as compared to the noble, romantic, or dull queen or princess she is companion to. A classic example is Racquel Welch in The Three Musketeers (1973); she plays the lady-in-waiting Constance, and all her gowns are low-cut to display her assets [pictured].
Louise seems to be thinking of a specific lady-in-waiting in a blue velvet dress, but I don’t know which one. The period film Elizabeth came out in 1998, based on the early reign of Elizabeth I; as they are studying her in Government that would seem the most likely candidate. However, there is no dress such as Louise describes, and although one of the ladies-in-waiting does have sex because of a particular dress, it ends in tragedy for her.