DOCBOOK: LegalNotice in Chunked HTML Navigation

Can anybody give a hand understanding how to customize the HTML
stylesheet so that it produces a copyright message in the chunked
navigation header or footer? Ideally, it'd also be a link to
LegalNotice.

Some of the mechanism already seems to be present in dbnavig.dsl around
line 229, but I'm a DSSSL newbie so it's still pretty opaque to me. Can
anybody explain what's going here and tell me if this is the best way to
be customizing the header or footer?

      (if %header-navigation%
          (cond
            . . .
           ;; LegalNotice only happens when %generate-legalnotice-link%
is #t
           ((equal? (gi nd) (normalize "legalnotice"))
            (default-header-navigation nd
              (empty-node-list) (empty-node-list)
              (empty-node-list) (empty-node-list)))

    Thanks,

    Bob