What about those clear belly covers (or shebaka)? Or possibly a sheer leotard?
Years ago I used to watch dancers at the local fair, many of which were quite heavy. Believe it or not they would use extra scarves which were tucked under the belt so they floofed over the top. It accented the hips making them larger and, while you never thought they were thinner then they were, widening the hips did de-emphasize the size of their bellies. I think that could also help cover up other things such as an ostomy bag.
Occasionally one of them would use a scarf like Shanazel's example "a" but most it was swag and floof. The middle of the scarf would swag across (usually) the back and the ends would be folded and tucked under then over the belt like fluffy "puppy" dog ears. Some did both, the shoulder part tucked into the bra strap so it wouldn't fall down. One tucked the end of the scarf into the shoulder, swaged across the belly, under the belt, and then further swagged and floofed. Most of them used at least 2 scarves so there were a lot of options and plenty of floof.That I a great idea! How did they tuck them in other places, like in the top?
I was always self-conscious about my tummy- even in those years when I didn't have one! To disguise it,
a. I draped a veil over one shoulder and tucked the ends into the hipband on the opposite hip.
b. I cut a hole in the center of a three-yard length of chiffon and wore it over my bedlah and skirt like an open-sided belidi dress caught at the hips by the hip band. (This was my favorite method of hiding a post-baby tummy. The hips were emphasized and the torso was disguised; an opaque version would easily hide an appliance.)
c. With the center of the veil at my back, I tucked the ends of the veil under my straps with the right hand end tucked on the left hand side and the left end on the right hand side so the veil crossed by body in the front. This was also nice for veil removal during a spin.
The next time I am on my desktop computer, I'll see if I can find photos from my classes. Alas, I don't have photos from the seventies. My then-boyfriend was also my photographer. After we broke up, he absconded with my photos and I was unable to get them back. The cad, right?
And don't forget flounce!Swag and Floof sounds like a TV series about a couple of New Orleans undercover cops.
b. I cut a hole in the center of a three-yard length of chiffon and wore it over my bedlah and skirt like an open-sided belidi dress caught at the hips by the hip band.
But since the original question was about Saidi dresses, the TL;DR answer is to go with a more Tahtib stylization and a folkloric costume like what Fifi is wearing below. You can always engineer the dress to make sure the "belt indentation" hits you at the best spot by gathering it there and attaching the sash to the dress on top of it. (Google "80's drop-waist blouson dress" to get the structural idea I'm talking about.)
I wasn't sure what had prompted the OP. It seemed like a bit of an odd question to ask if someone had a lot of dance experience and had had an ostomy for a while, so I guess I assumed since she was talking about her dresses no longer fitting, she was new to the ostomy as a dancer. I didn't mean it to come across as talking down or [___]-splaining, and if it did, I apologize.
I remember a similar question coming up years ago on Bhuz, and you never know who is lurking or will find a thread in the future. There surely are others out there who wonder about belly dancing and ostomies, or who have similar questions about a costuming need to cover an abdominal issue, so maybe it's still useful...