Customisation of title page not working in DocBook SGML

[Jeremy Malcolm <Jeremy@Malcolm.id.au>]

I posted this message to comp.text.sgml, but I didn't receive any
replies so I'm reposting to this list.

I am trying to customise a book title page, and my driver file contains
this:

;; add bibliomisc element to title page
(define (book-titlepage-recto-elements)
  (list (normalize "title")
        (normalize "subtitle")
        (normalize "graphic")
        (normalize "mediaobject")
        (normalize "corpauthor")
        (normalize "authorgroup")
        (normalize "author")
        (normalize "editor")
        (normalize "bibliomisc")))
(mode book-titlepage-recto-mode
  (element bibliomisc
    (make paragraph
      font-size: (HSIZE 6)
      font-weight: 'bold
      (process-children)))
)

The title page does not include my bibliomisc element, though (other
changes made in the same file do work though, so I know it's being
processed).  What am I missing?

TIA