I did an under-bust corset as part of my Santacon outfit a few years ago, did it from a Simplicity costume pattern. The front was actually 2 independent pieces that lace with ribbons to each other and to the rest of the corset. Maybe, with 3 rows of lacing, you'd have plenty of give. You could even lace in larger pieces once the smaller ones start looking like tiny islands on the ocean of belly.

You can sort of see the corset in this photo.
people.tribe.net/4a278b27-...bf403177ed

I would check into weather or not that is recommended. From what I recall corseting a pregnant belly is not recommended because restricting the abdomen can bring on contractions.

you may want to opt for a high waist ed vest with 2-3 buttons under bust and a full skirt that goes over belly, Or over belly pants with a pleted elastic wais and tapered legs. A short cape over the whole ensemble would also add steam punk flair.

I've heard it's a bad idea to corset pregnant bellies, too. A light bustier with stretchy panels, and removable stays, at least in the front, might solve the problem. Sewing the stretch into a corset would be a PITA for me, but if your experienced it shouldn't be too hard--I've had friends do stretch bustiers. I think it'd be cool to do something that really accentuates your shape--white with black piping or ribbons along the stays (or vice versa)? Maybe gold and black? hmmm. Simple and striking would be awesome looking. Use long laces and don't lace tighter than it needs to be to stay on. Also, maybe you could add lacing in other places than the back--might be nice to have lacing at the front/sides to give you a little more adjustability.