SPIN_py is a Python module that wraps the SP SGML parser. It does not follow SAX, nor DSSL, but it performs a similar task. It has code to build (and pretty-print) trees from C++ or from Python, and it has an interesting object model, with virtual C++ objects, based on PyObject_HEAD.
refdb-mode is a minor mode for Emacs that implements an interface to RefDB, a reference management and bibliography tool for SGML, XML, LaTeX, and Muse documents. It integrates nicely with psgml, nxml-mode, AucTeX, and muse-mode. refdb-mode provides the complete functionality of the RefDB command line clients from a graphical interface. You can add and edit references, notes, and styles, run queries, and insert the results as citations into your DocBook, TEI, LaTeX, or Muse documents. You can also look up cited references in the database right from your document. You can create, transform, and view these documents with a mouse click. This amounts to an integrated authoring environment for markup languages with reference management and bibliography support.