Overriding Panels' default category structure
"Whaddayamean you can override functions starting with 'template' like you can functions starting with 'hook'?" I ask...

I've finished my organizer and will post it as a sandbox module if my boss lets me. I ultimately just copied the source of template_preprocess_panels_add_content_modal() from panels/includes/add-content.inc and changed what was passed on -- $vars['categories'], $vars['categories_array'] and $vars['root_content'] were particularly important.

Surprised nobody answered this -- would've been an easy 10 points to just say "Yeah, template_preprocess functions work a bit like hooks in that you can insert your own version higher up the chain just by naming it a certain way..."