Laurent Godard announced on the docbook-apps list about the freely-available ooo2dbk tool to transform OpenOffice.org documents to DocBook:
ooo2dbk is a tool allowing the transformation of OpenOffice.org documents to docbook. Based on Eric Bellot's work, it has been widely extended and allow enhanced exporthttp://indesko.com/sites/en/downloads/ooo2dbk___generating/view
Supporting XML docBook V4.3, it allows to generate both article and book documents A lot of features are available such as handling preface and appendix elements, metadatas as OOo user fields also
ooo2dbk is an independant script allowing then batch processing of OpenOffice.org documents
I contributed some code to the xmerge project a few years ago, but haven't had time to play with it for a while. It would be interesting to see where the filter is at now, and contrast it with the ooo2dbk tool...
See also: DocBook