How about extending the functionality a bit:
in both of these files:
/includes/templates/YOURTEMPLATE/templates/tpl_index_default.php
/includes/templates/YOURTEMPLATE/templates/tpl_index_categories.php
at the bottom, but above the final </div> add a line like this:
PHP Code:
<?php include($template->get_template_dir('tpl_my_index_content.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_my_index_content.php'); ?>
Then make a new file at:
includes/templates/YOURTEMPLATE/templates/tpl_my_index_content.php
and in that file put your custom HTML code.