Mine is much simpler

(use-package howm)

Although I am the one who use it with org-mode too.

I've done it using prop-line feature of emacs and my own interactive, for today it is only interactive, function howm-insert-prop-line:

    (defun howm-insert-prop-line ()
  "Activate major mode and modify the file so that this mode is activated
  automatically the next time it is opened"
      (interactive)
      (howm-mode)
      (let*
   ((modes (mapcar #'cdr auto-mode-alist))
    (mode-name (completing-read "Choose major mode: " modes))
    (mode (intern-soft mode-name)))
 (unless (or (null mode)
     (eq mode major-mode))
   (funcall mode)
   (howm-mode)
   (add-file-local-variable-prop-line
    'mode (intern (string-trim-right mode-name "-mode\\'"))))))

It is in github repo, too although it is not yet finished:

https://github.com/artsi0m/howm-use-any-markup

My problem is that I can't combine well nested interactive+let or let+interactive, so it is only as interactive function for now.