Embed HTML in Message box

FWIW, I found that adding the following two lines to CKEditor's global profile >> visibility settings >> fields to exclude, disabled CKEditor on both the admin and user pages. There must be something similar for the other WYSIWYG editors.

admin/settings/print*.edit-*
printmail/*.edit-*

I wonder if there is a way for an app module to tell the WYSIWYG editors to leave certain textareas alone i.e. plaintext only.

That works great, at least for CKEditor. I don't have a setup to test TinyMCE. I had to modify the 'txt_to_addrs' textarea also to get the same result as #2. I added the extra lines as shown below. I didn't try to find all the textareas in the admin pages, but I would recommend disabling wysiwyg there also.

  $form['txt_to_addrs'] = array(
    '#type' => 'textarea',
    '#title' => t('Send to'),
    '#wysiwyg' => false,
    '#rows' => 3,
    '#resizable' => FALSE,
    '#description' => t('Enter multiple addresses separated by commas and/or different lines.'),
    '#required' => TRUE,
  );
. . .
  $form['txt_message'] = array(
    '#type' => 'textarea',
    '#title' => t('Your message'),
    '#wysiwyg' => false,
    '#rows' => 6,
    '#required' => TRUE,
  );