I don't do jumpers etc. at all but you cast off different places in different rows right? Wouldn't that be the same regardless of whether you're ribbing or not? Same as how you would do it flat? Have a look for a tank pattern in ribbing with a curved neckline and see how they do ?
You want the shape of the collar, right? Because the "smooth" edge is picked up and knitted, regardless of the body stitch. Just do your normal shaping. It doesn't matter if the body is ribbed. The smoothe edge in the pic looks to me like a hemmed edge. I hope this helps!