I had a client with a similar issue, and so I wrote a DocBook-to-DITA XSL
transform to use as a preprocessor for that output. They used a
well-defined subset of DocBook and so the stylesheet did not have to account
for all DocBook elements and combinations. As far as I know, they are still
using that in production. I can't share that stylesheet with you, but it
was pretty straightforward to develop.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net