My house has a 4:12 pitch with 2x8 rafters, full hips, so I've actually thought about what you are suggesting. A sort of "large crown molding" around the exterior walls, but then I thought about the retrofit cost of that at an existing home vs the energy savings and realized I was drunk. But in new construction, that's a different story. Of course at that point, why not just get energy trusses. It seems to me that would be a reasonable alternative if you can provide the R-value and the envelope equivalent to conventional.