In Views areas (header, footer, empty-text) are pluggable, this means you can write your own php logic to place whatever you want.
function yourmodule_views_data() { $data['views']['collapsible_area'] = array( 'title' => t('Collabsible Text area'), 'help' => t('Provide collabsible markup text for the area.'), 'area' => array( 'handler' => 'yourmodule_handler_collapsible_area_text', ), ); }The second step is to write this handler. Therefore create a file called yourmodule_handler_collapsible_area_text.inc and add it to the .info file of your module. Then add content to your area file like this:
class yourmodule_handler_collapsible_area_text extends views_handler_area_text { function render($empty = FALSE) { // Here you just return a string of your content you want. if ($render = parent::render($empty)) { $element = array( '#type' => 'fieldset', '#title' => t('Title'), '#value' => $render, ); $output = theme('fieldset', $element); return $output; } } }As on every handler you can add options so you can configure the behavior. If the area isn't shown yet in the views interface, please clear the cache :)