I don't know of any CMS that allows you to do this out of the box. CMS's are usually made to create webpages after all.
Drupal
This can be done with Drupal, but it will require some work, depending on what type of XML RDF documents you need to generate, this can be a greater or lesser work load.
Since all markup is defined by theme functions and templates, all you need to do, is to overwrite the default ones. If all you want, is to create this special output for raw content - called nodes in Drupal - you would only need to create a customized node template, and a special page template, which creates the surrounding markup like doctypes etc.
If this is the case, you shouldn't need much extra work compared to a special design node, like products in a shopping site. The context module can help you managing which page template to use for different types of content/urls.
So with the modules available and Drupal's great flexibility, could be done without too much work. My guess is that Drupal would be the best choice, since for such a thing, flexibility is critical, the thing Drupal does better than all other CMS I've seen.