I've run across some additional requirements for allowing subtitles in
sidebars. This would require a change to the sidebar content model for sidebar.
subtitle is already allowed in:
These elements contain subtitle: acknowledgements, appendix, article, bibliodiv,
biblioentry, bibliography, bibliomixed, bibliomset, biblioset, book, chapter,
colophon, dedication, glossary, glossdiv, index, indexdiv, info (db.info), info
(db.titlereq.info), part, partintro, preface, reference, refsect1, refsect2,
refsect3, refsection, refsynopsisdiv, screenshot, sect1, sect2, sect3, sect4,
sect5, section, set, setindex, simplesect, task, tocdiv.
Seems like an oversight for sidebars. This particularly affects the Publishers SC as well.