適当に作ってみたよ。 M-x hiwin-mode

(defvar hiwin-face 'highlight)
(defvar hiwin-window nil)
(defvar hiwin-buffer nil)
(defvar hiwin-overlay nil)

(defun hiwin-highlight-window ()
(unless (or (eq (selected-window) (minibuffer-window))
(and (eq hiwin-window (selected-window))
(eq hiwin-buffer (current-buffer))))
(setq hiwin-window (selected-window)
hiwin-buffer (current-buffer))
(unless (overlayp hiwin-overlay)
(setq hiwin-overlay (make-overlay 1 1 nil nil t))
(overlay-put hiwin-overlay 'face hiwin-face)
(overlay-put hiwin-overlay 'after-string
(propertize (make-string 100 ?\n) 'face hiwin-face)))
(move-overlay hiwin-overlay (point-min) (point-max) hiwin-buffer)
(overlay-put hiwin-overlay 'window hiwin-window)))

(defun hiwin-mode ()
(interactive)
(if (overlayp hiwin-overlay)
(progn (remove-hook 'post-command-hook 'hiwin-highlight-window)
(delete-overlay hiwin-overlay)
(setq hiwin-overlay nil hiwin-window nil hiwin-buffer nil))
(add-hook 'post-command-hook 'hiwin-highlight-window)
(hiwin-highlight-window)))