The core of the tool will take as input and output abstract representations of the document-class. Those data structures will be defined by a generic schema-handling library.
The input to this tool core will be produced by a modular schema parser, allowing to plug parsers for new notations for schema descriptions.
The output of the tool core will have to be transcribed into a usable syntax (which may be other than the input syntax).
Output of differential customization layers, like those manually written for DocBook, may be supported using additional output modules. However, as this way of customization probably only exists to simplify the manual work this tool hopes to render useless, it's not clear to me at this point why it should be supported, and I don't plan to put any work on this myself.