I think you mentioned before eventually enabling templates for email, so this is just an idea to consider.

We have a set of well formatted HTML email letters I keep in the Joomla content articles, so I added a button in the Mass Mailer to fill the editor message block with the email. This example is for our welcome email but I will eventually have a select list. I had to solve several technical issues to get this to work.

JModelLegacy::addIncludePath(JPATH_BASE . '/components/com_content/models', 'ContentModel');
$model = JModelLegacy::getInstance('Article', 'ContentModel');
        $articleId = 232;
$article = $model->getItem($articleId)->introtext;
$article = str_replace(array("\r\n", "\n", "\r"), ' ', $article);
$article = str_replace('\'', '‘', $article);
$article = "'".$article."'";
$button = '<button class="button" onclick="clickme();">Welcome Letter</button><script>function clickme() {  tinymce.activeEditor.setContent('.$article.')}</script>'."\r\n";

and $button is put before the div on line 125 ('MASS_MAILER_MESSAGE_LABEL') of administrator/components/com_comprofiler/view/view.users.php

Of course, the HTML email article contains the replacement parameters, like [NAME], appropriate for the Mass Mailer.

Thanks. I will consider that when I've made it work more generically.