That said, I am wondering if the expressive power of freetype internal vector
code could satisfy the requirements of the font svg rendering. Because that
would reduce the external dependency to some xml parser, then some internal
freetype code would "translate" this font svg directly into internal freetype
vector code.
FreeType historically was a colorless rasterizer and only returned a pixel coverage map, which could then be colored and blended by a client application. As of the last version, FreeType can now add color according to CPAL/COLR tables, where each layer is blended with a monocolor. To support SVG, tt is a matter of implementing color gradients, which is not that hard. Some work needs to be done to implement data structures for complex colors and properly isolate the blending code into a dedicated blender-rasterizer.