CONF-9291 is implemented. The documentation of this plugin type is here, but private to atlassian-staff:

http://confluence.atlassian.com/display/DOC/Renderer+Component+Plugins

When 2.8 is released we should make it public and link it from the table on its parent page

As the document makes a pretty strong effort to discourage people from writing said plugins (documenting the renderer to the point it was hackable by third parties would be a significant task in itself), I don't think we should cover this in the release notes. The only reason I documented it in public was that people would find it anyway and ask where the doc was.