Cool! Nice to know that I'm not the only person trying to implement K5 from scraps and speculation.
Since you are playing in JS, you might also have a look at c.js (http://kx.com/q/c/c.js), which is the tool KX uses to translate between JS objects and k objects through web socket IPC with a k process (http://code.kx.com/wiki/Cookbook/Websocket). Might be useful to see how Arthur envisioned the k data structure in JS. Also, it would be kind of interesting to have a k5 process running in the browser talking with a k4 process running locally in the 32bit trial...