I played around with WebSockets and wrote a new tool: https://github.com/doyensec/wsrepl
It's an interactive REPL interface like websocat, but it's meant specifically for pentesting, not debugging, and it's easily extensible in Python (while still retaining REPL interface). In future releases I'd like to expand the extensibility by adding declarative style configuration (the ultimate feature would be something like what Burp's Autorize plugin does, but for websockets).