Update: after the end of the contest, I changed the neckline and waist to elastic inside the casings, instead of ribbon. This is more comfortable, sits better inside the very curved neckline casing, and means that the waist ribbon doesn't have to be tied/untied every time the gown is put on.
The neckline casing was actually split into two rows by adding a line of stitching. Then I used two rows of narrower elastic, so that I could use a shorter length in the row closer to the edge, better fitting the curve.
The bows are now pre-tied ribbon tacked on.