Currently, I have an basic generator running (exactly the way you described). I can successfully decode a CodeGeneratorRequest, for which I had to write all decoders by hand initially. I am now working on generating modules from the request. Here I got delayed as a found out the API was missing a Decode.lazy for recursive structs (like DescriptorProto). Once it is mature enough I will make it public as it will support using elm-protocol-buffers.
I am not sure what you are referring to here: generating code or de/encoding ProtoBuf JSON? Feel free to ping me on slack @eriktim.