Inserting LaTex expressions using a filter fails

[Mart van de Wege <mvdwege@gmail.com>]

I'm trying to replace U+00BD in an org buffer with \sfrac{1}{2} during
export to LaTex, and obviously I'm doing something wrong, or I don't
understand the documentation.

I use the following code to set up the filter:
#+BIND: org-export-filter-item-functions (latex-replace-half)
#+BEGIN_SRC emacs-lisp :exports results :results none
  (defun latex-replace-half (text backend info)
    (when (org-export-derived-backend-p backend 'latex)
      (replace-regexp-in-string  "½" "\\sfrac{1}{2}" text)))
#+END_SRC

Down the line in the document there is this example line:

   - 1½ eetl. gehakte peterselie, ½ eetl. azijn, 4 eetl. olie

Or this one:

   Laat de gewassen en gebroken vermicelli met de foelie ½ uur in de

But neither of them get replaced. I tried plain-text, item, and
final-output filters.

What am I missing?

Mart

-- 
"We will need a longer wall when the revolution comes."
--- AJS, quoting an uncertain source.