Cross posting a comment from the
JAVASERVERFACES_SPEC_PUBLIC-971 ticket because it is relevant:
I have implemented a similar concept in JSF as well. In my experience you need a hierarchy of message resource bundles as well. You have a global system bundle for the majority of messages, and a template/brand/theme specific resource bundle that can override any of the global system messages, as well as add additional messages needed by the template/brand/theme (for their header, footer, etc.) You also need to take into account images that have text on them and need to be localized.
Also, we have created a .xhtml facelet file for every screen of our application in each template/brand/theme directory. It's not just the header/footer that changes... the way they want to lay out panels looks different, their placement of buttons, etc. We need to make our application look exactly like the customer's existing website so their customers don't realize they have gone to another site for our particular workflow. Every customer is totally different.