My first thought is that if you can't easily make a DocBook structure
deliver your content on the correct page of a PDF, you could generate
the back cover content separately and use a post-processing step to
rearrange the final PDF.
I might succumb to that sort of solution. I was just thinking that since you
can customize your front cover completely, maybe there was a way to do the
same for the back cover? I don't need to do anything fancy and could do with
a few XSL pushups in a template.