Ummm... i know it's an anime, but the actual dress and time is medivial China being portrayed, so none of them are wearing kimono, just stylized Chinse dress, and actually well portrayed. The item worn by all people, men, women, child, rich and poor is called a Hanfu. http://library.thinkquest.org/05aug/01780/clothing/history.htm
I got really lucky that the local history museum has a REALLY nice display right now of emporial Hanfu robes, several of which are very close to what Ryuuki wore in the show. The mere fact that some of them are 400 years old just blows my mind cause they are still in great condition and I took measurements off of the female pieces and I could wear them!!!heeheheheh
The thing you are regarding as a corset, is actually an overskirt displayeed simimular as to an Elizabethan forepart. From what I can gather, the "skirt" is wrapped around twice and secured by a decorative cord. It also should go down past the floor. Being an entertainer, Kochu would have been a little free with her dress and be more flamboyant. I know that Japanese companions had a special dress code to deffer there profession(ironically it is used as the icon for Arizona Green tea, big gallon jug)and had restrictions of how they could tie obi's while "on duty."
for the most part I am cheating and making them zip up on the sides. I don't get much say in the matter.... If you want any other help, of info, or f you need me to get pics of the hanfu on display, let me know,K?