DOCBOOK-APPS: XSL stylesheets: adding elements via customization layer

I'm trying to extend the XSL stylesheets to cater for our extensions to
the DocBook DTD. I have elements at the same level as <sect1>, which are
to be treated the same way as <sect1>s. I've managed to get the TOC in the
HTML Help appearing, but I'm getting this error while processing with
Saxon 6.4.3:

No template named "commandlineprogramdef" exists in the context named
"section-title" in the "en" localization.

I've searched the entire stylesheet installation tree, and can't find
where to define this. The only places where context section-title appear
are in common/gentext.xsl and en.xml. The error message is generated in
l10n.xsl.

I've overridden all code that process <sect1>s to include my elements, and
I can't get rid of this message.

Any ideas where to look or how to fix this?

Any hints on adding support for new elements to the stylesheets? I get the
feeling I'm reinventing the wheel here...

TIA,

Gershon