The GLTF Writer doesn't support the addition of feature attributes to Nodes/Groups in the written GLTF files.
The GLTF schema allows for both a "name:string" property and an "extras:map/object" property for both Nodes and Meshes. This could be used in combination with the schema definition/user attributes of the writer. Perhaps with options to apply at Node or Mesh??
I currently use a workaround to this with the FeatureWriter using GLTF/json and then on write, read this back in with the JSON Reader file, merge with the original feature dataset using JSON Templating. I then write this to local disk and compile the result as a GLB (with Draco compression) from the command line. (This last step could conceivably be a python script)
It would definitely be beneficial to go straight to the GLB from the original writer.