Mechanism for inserting custom content in pre-existing code

I've been trying to figure out a mechanism for allowing XWiki Applications to alter the content of existing interface elements, like adding a new tab in the admin mode, or a new page in the global preferences, or new menu items, and I think that the best solution is to use an Overlay class. Here's how it would work (inspired by the awesome XUL model):

• an overlay object registers a piece of content that should be added inside a container, identified by an id.
• the added content can be inserter either at the start or at the end of the container, with an optional neighbor component (e.g.: inside #globalmenu, at the end, if possible before #watchmenuentry). If possible means that if that component exists, respect the required ordering, but if doesn't exist, don't fail.
• we provide two functions, onStartId and onEndId, which automatically detect what are the components that must be also added
• each application provides such overlay objects in a document inside the .xar archive

This makes it easy to extend some content, without actually modifying it, but requires massive changes in the existing template to replace <div id=..."> elements with onStartId() calls.