extending case-fold-search to remove nonspacing marks (diacritics etc.)

[Ted Zlatanov <tzz@lifelogs.com>]

https://emacs.stackexchange.com/questions/7992/how-to-search-an-arabic-word-in-text-without-its-diacritics-accents
suggested it would be useful if diacritics were ignored when searching
for text in various situations. This is similar to `case-fold-search'
but more generic. Here's what I suggested as the answer at the ELisp
level:

#+begin_src emacs-lisp
(defun kill-marks (string)
  (concat (loop for c across string
                when (not (eq 'Mn (get-char-code-property c 'general-category)))
                collect c)))

(let* ((original1 "your Arabic string here")
      (normalized1 (ucs-normalize-NFKD-string original1))
      (original2 "your other Arabic string here")
      (normalized2 (ucs-normalize-NFKD-string original2)))
  (equal
   (replace-regexp-in-string "." 'kill-marks normalized1)
   (replace-regexp-in-string "." 'kill-marks normalized2)))
#+end_src

This would probably be useful for other languages, not just Arabic. But
implementing it for users so it works like `case-fold-search' (you just
set something in Customize and all search commands DWYM) seems much
harder. Does anyone have suggestions? Maybe some defadvice magic? Or is
it not possible?

Thanks
Ted