I've made significant progress on NNA (FBX extension system).
You can now define significant parts of a #VRChat and #VRM avatar in #Blender, and have it just work in #Unity!
I've written a Blender extension to make it easy to create these definitions^^
Blender Extension: https://github.com/emperorofmars/nna_blender
Unity Importer: https://github.com/emperorofmars/nna_unity