wysiwyg in rules text field
Yup. You can use hook_FORM_ID_alter() to change any field into text area into a WYSIWYG field:

function MYMODULE_form_rules_ui_edit_element_alter(&amp;$form, &amp;$form_state, $form_id) {

  //Set the type to text format
  $form['parameter']['message']['settings']['message']['#type'] = 'text_format';

  //Replace 'full_html' with 'filtered_html' or any other input format that has a WYSIWYG profile
  $form['parameter']['message']['settings']['message']['#format'] = 'full_html'; 
}