You could look into some Art Nouveau images for ideas, since that's the inspiration for those costumes. (google image search: http://images.google.com/images?svnu...rl&btnG=Search
It seems that most of the designs are fairly basic long dresses, with interesting touches of jewelry, like the chest decorations.
You could also look at Fuu's other art nouveau artbook designs:
(can't see much of her in this one... but it might help for ideas)
You might also look at Emeraude's or Presea's dresses... those are very similar.