This question is closely related to
Is there a way to programmatically determine if a font file has a specific Unicode Glyph?
However, I am looking for a solution that does not use the Windows API. Freetype in particular looks promising, and I was wondering if anyone familiar with it could provide pointers.
Basically, I'd like to, given a font, calculate a mapping from character map to a sequence of codepoints without glyphs for that character map.