That's a bit harder. You could try something like this:
require_once QA_INCLUDE_DIR.'qa-page.php';
require_once QA_INCLUDE_DIR.'qa-theme-base.php';
$themeclass=qa_load_theme_class(qa_opt('site_theme'), 'custom', qa_content_prepare(), 'anything');
$themeclass->nav_main_sub();
I haven't tested this code so there might be issues.