This is ONE possible fix, not necessarily the best. It's hacky because it takes advantage of the fact that the resolver valve looks in the layout directory before the screens directory. I added a file called RSS.vm to the layout directory which just contains the xml header, and a file called RSS.vm to the screens directory, which contains the actual RSS XML.
The better fix would be for ViewContextPopulator to allow overriding the default layout on a per-view basis. However, since this is the only place (so far) in continuum where a non-standard layout is used, maybe that's not worth the effort.