I just googled this gown (not familiar with the character), and it is gorgeous! I think you're right to start out with a bunch of extra long panels. Remember to keep the proportions of a non-ruched skirt if you are drafting your own pattern or if you're extending the length of a regular hoop skirt pattern. I would think that you could do the ruching two ways. One option would be to sew a channel alongside each seam allowance and then run cording through each channel. You could then gather the skirt up evenly along the cording and secure the gathers along the seamline. This could be accomplished by hanging a piece of twill tape inside the dress and then tacking along the seamline to the tape. In this version, you'd do the gathering after the main body of the skirt is constructed and (hopefully) take the whole thing up evenly at one time.
Another approach would be gathering each panel evenly along its length before sewing them together into a skirt. This would probably be pickier, but might yield a "puffier" finished product.
Hmmm... there are probably other ways to go about it, but those are some ideas for now...