There are a lot of really nice, inexpensive, and lightweight foundation garments that work wonders on smoothing out belly bulge. I wonder if you tried something like that under the skirt?
If part of the costume is a bare midriff that might not help ... but then again, if you got something that matched your skin tone it might not be obvious. Or try a fun colored leotard and people will think it's part of your look.
You could also see about altering the waistband so it hits in a slightly different spot. I know with me, there are two points where a waist band can hit that will cause my fat to bulge like crazy but everywhere else actually looks relatively normal.
Just do what helps you feel comfortable and keep dancing! If you feel good, you will look amazing!!!
Journey Started - 7/8/2013 @233; ONEderland - 9/26/2013; Lowest weight - 12/20/2014 @177
Journey Restarted - 9/10/2014 @200