I've never used a hollow rib or put a rib on a swamped barrel, but I don't think I'd want it parallel to the bore. Seems to me that it would look best to have the rib the same height at the muzzle and breech end and file it to fit the swamp in between if not just follow the swamped profile. Imagine the profile of the "web" of a fullstock between the ramrod and the barrel. Perhaps you could find pictures of originals and learn how it was done originally.