If fuller satisfaction turned out to not be possible, I would say just file the seam smooth and hope for the best. But a rim joint that is shifting isn't a good thing. If there is no sign of abuse, Campy should replace that wheel for you. I suspect it is a pinned joint rather than welded. Is that right? Actually all my rims are that type, so I'm not casting aspersions, just saying that would make the most sense.