This may just be adding noise to the discussion, but there’s a thing I hacked into CSL-M that allows content to be embedded in a term. If that form is required everywhere for pages, I guess that with that it could be expressed like this:
<term name="page">
<single>%s f.</single>
<multiple>%s ff.</multiple>
</term>
(only if it’s not a form optionally chosen by the document author though)