Here's a breakdown since I recently went through the process of dropping something into WHM.
Template file goes into: /usr/local/cpanel/whostmgr/docroot/templates
Wrap it with this(you can remove scripts / stylesheets if you aren't using any external ones):
stylesheets = [
scripts = [
<!--HTML/Template CONTENT -->
[% END %]
This uses the latest WHM Master template, the _defheader is something we're moving away from. Now when we call Templates we have to use Cpanel::Template and it usually looks like this...
'template_file' => 'sometemplate.tmpl'
The breadcrumbs / heading is based on the /usr/local/cpanel/whostmgr/docroot/themes/x/command2 file which I believe is modified when you register the plugin and appropriate files are regenerated. My knowledge of WHM plugin development is a little fuzzy so I'll try and investigate for you.
edit: Here's a pastebin link since the formatting gets destroyed in this: http://pastebin.com/Fqq3MUvp