Emacsを立ち上げっぱなしにしてるとhowmのバッファがどんどんたまって
いきますが、切り替えのときに日付を頼りに (2004-12-17-203050.howm とか)
しないといけないのが面倒なので、こんなの作ってみたんですがどうでしょう。
既出だったらすみません。

(global-set-key "\C-c,b" 'howm-switch-to-howm-buffer-by-title)
(defun howm-buffer-title-alist ()
"Return alist of titles and buffers in howm mode."
(let ((result nil) (buflist (buffer-list)) (count 1))
(save-excursion
(while buflist
(set-buffer (car buflist))
(save-excursion
(when (and howm-mode (not (= ?* (aref (buffer-name) 0))))
(let ((title "(no title)"))
(goto-char (point-min))
(when (re-search-forward "^= *\\(.*\\)$" (point-max) t)
(setq title (concat (format "%d. " count) (match-string-no-properties 1))))
(setq count (1+ count))
(setq result (cons (cons title (current-buffer)) result)))))
(setq buflist (cdr buflist)))
(nreverse result))))

(defun howm-switch-to-howm-buffer-by-title ()
"Switch to howm buffer with completion by title."
(interactive)
(let* ((buffer-title-alist (howm-buffer-title-alist))
(selected (completing-read "Switch to: " buffer-title-alist nil))
(buf (cdr (assoc selected buffer-title-alist))))
(if buf (switch-to-buffer buf))))