OK, I think I'm on the path now. I was able to create the skin by making derived sketches in the new part ... one sketch per plane per rib. I wasn't able to derive a sketch into a plane within a 3D sketch, but a series of 2D sketches worked fine. I could then setup my guide paths and do the loft. I still need some practice to make it clean, as you can see in the attached image, but I think I know what needs to be done.