I would take a look at XPontus, it's an open source XML editor. It should have support for DocBook: Release Notes.
Overview
XPontus XML Editor is a simple XML Editor oriented towards text
editing. It can perform validation(DTD, XML Schema, Relax NG, Batch
XML validation), XSL transformations(HTML, XML, PDF, SVG), schema/DTD
generation, XML/DTD/HTML/XSL code completion, code formatting and much
more. The brand new version is now plugin based to solve some
limitation issues.
XPontus aims to become the free alternative to commercial XML IDES
such as XMLSPy or Oxygen XML Editor, or Exchanger XML Editor without
the Entreprise features for now. The application tries to provide you
most of the features you would expect to see in a fully bloated XML
IDE, in a familiar and conviviable manner.
