As pointed out by greg-449, Kepler Service Release 1 was recently added to the Kepler p2 repository http://download.eclipse.org/releases/kepler/. So the new versions of org.eclipse.emf.ecore.xmi etc. are in fact part of the Kepler repository.
The fact that Tycho cannot download the new version of the artifact seems to be a problem in your Nexus p2 repository proxy setup. The artifact org.eclipse.emf.ecore.xmi_2.9.1.v20130827-0309.jar is available in the original repository (both logically according to the p2 index and physical). From my experience, the Nexus p2 repository proxy doesn't work reliably with composite p2 repositories (like Kepler).
There is the option to only use the June release version of Kepler: Just replace the Kepler p2 repository URL by http://download.eclipse.org/releases/kepler/201306260900/. This p2 repository is one of the sub-repositories that make up the full Kepler repository. (To see all the sub-repositories in the Kepler repository, check out the repository's compositeContent.jar.)