Cool project! I've done similar things with Kurento acting as a WebRTC peer to proxy WebRTC --> RTMP before but this is a lot more lightweight.
One thing I'd love to see at some point is a project that compiles FFMpeg's transmuxing code to WASM such that the WebM --> FLV conversion could be done client-side. That way the server-side portion would be as simple as proxying the WebSocket to a TCP socket for the outgoing RTMP traffic.