You need not only geometry/tess shaders but also compute shader and signed framebuffers. In theory it would be possible to work around the lack of compute shader (making the minimum requirement GLES 3.0), but it would involve multiple passes and I'm not optimistic about the performance in that case.
Would the fallback be better placed into WebRender or Pathfinder? It would be nice to be able to know that you only need to import one library for font rasterization, regardless of the hardware you're running on.