HarfBuzz seems like it ought to fit the bill. It does come with some freetype-integration code, but you don't have to use that; it's fairly straightforward to u

HarfBuzz seems like it ought to fit the bill. It does come with some freetype-integration code, but you don't have to use that; it's fairly straightforward to use harfbuzz shaping in conjunction with Quartz2d or DirectWrite glyph rendering, for example. I imagine using it with libschrift would be similar.