Ribbing is stretchy by nature, so I think the idea is that it should be able to accommodate your curves. To make sure it'll work, how about trying this -- when you swatch, measure your gauge when relaxed, and measure again while stretched. If the relaxed-to-stretched ratio is lower than your waist-to-hip ratio, you should be okay. (Someone please confirm this. I think that's right.)
If that doesn't work, I know
I've seen directions on how to increase while in ribbing, but I can't for the life of me find them now. It was probably in one of the books I borrowed from the library and has since been returned... I think the result was kind of weird looking, though. Like you ended up with little Y's branching out where one rib separated and divided into two, if that makes sense. I suppose if you did four of them strategically like darts it would be okay. Hopefully you won't need to worry about it, though.