[Fontconfig] Is code point substitution inside the same font possible with fontconfing?

[Vasile Gaburici]

Briefly, I want to alter the code point of a request before passing it
to the font library (freetype2 in my case). The recoding that needs to
happen is from U+021A/B to U+162/3. This is quite useful for new
Romanian documents rendered with old/free Type-1 fonts (details here:
https://fedoraproject.org/wiki/L10N/Tasks/Ro_fonts#Type-1_fonts_need_an_additional_mapping_from_U.2B021A.2FB_to_U.2B162.2F3)
Microsoft's Uniscribe does this automagically for type-1 fonts (but
not for OTF/CFF ones, which handle stuff like this with GSUB tables).
I'm trying to implement this behavior using fontconfig, but after
carefully reading the manual for the config file, it doesn't seem
possible. Do I have to write a C hack, or am I missing something?