If I have two editor tabs open as two browser tabs they should already be able to talk if they're from the same origin, which different editor tabs would be.

The JS code won't be able to modify the browser itself, but neither can ELisp modify the actual Emacs binary code. The customizability in both cases is really coming from an "OS" being implemented in user-controlled scriptspace. You'd have just as much ability to customize that HTML-and-JS OS if it was running in the browser as you have to customize Emacs I think...