Having "one XML to rule them all" would be a pain to maintain, since OEMs come from a variety of people.
I think some sort of override system might be nice though. More specifically...
If there was an "override" folder that would always be processed last
Rather than error out when hitting duplicate UUID's, the tool would override the former values associated with that UUID.
Then we could have an easy mechanism to add personal customizations to existing OEMs, without needing to manually edit them with each Kitchen release.
Some of this is possible today using differing UUID's, but making sure your UUID is higher so it is processed later. But needing to fiddle with the UUIDs seems more difficult to me, and doesn't allow certain actions like deselecting an OEM package.
One missing function in either case is the ability to remove files from an other OEM without physically removing from the other OEM.