My Rufio crossplay has a bare stomach as well, and I am pretty damn curvy especially around my butt/thighs. My oufit was readily made to hide my curves though. What sort of outfit were you thinking of, and is there one that can hide curves via bagginess? Can you sneak a jacket in and have it still be accepted as canon?
What helped a lot for me was Rufio's bulky jacket which overhung my sides and hid the shape of my waist. I also wore my pants lower, but my belt sat on my hips to hide the curves there and gave the impression of a longer torso.
In the mirror, practice sucking in your core (abs). You'll find that it pulls your chest down and tucks your butt under giving the illusion of less curves.
Posing in ways that hide the outline of your waist may help, like crossed arms. Just make sure it doesn't emphasise your chest.
Padding in the shoulders and posing with arms apart to give the illusion of wider shoulders/torso and lessen the appearance of hourglass shape. Practise in the mirror and see what works for you.
My Rufio crossplay demonstrating the above: http://sphotos-c.ak.fbcdn.net/hphoto...05723399_n.jpg
Hope it helps.
EDIT - ignore the slipped binder in this shot, this has a good impression of how the belt sat to give the impression of less curves. http://sphotos-h.ak.fbcdn.net/hphoto...34070446_n.jpg