There is such a tool out there. I've used it for a while. It used to be available in Debian and was named sgml2X or docbook2X (I think the latest is more likely to be it).
Dans une Potato: __ Opt doc docbook2x-do <none> 0.5.1-6 HTML docmentation of the docbook2x packages __ Opt text docbook2man <none> 0.5.1-6 Perl SGMLS spec file to convert DocBook SGML docs into troff __ Opt text docbook2texi <none> 0.5.1-6 Perl SGMLS spec file to convert DocBook SGML docs into Texinfo
(même versions dans Sid et Woody apparemment)
-- Benoît Sibaud