Monkey-patching for postMessage the way it's used as a scheduler would be something like this:

class MessageChannel extends globalThis.MessageChannel {
  constructor(...args) {
    super(...args);
    this.port1.__channel = this;
    this.port2.__channel = this;
    patch(this.port1);
    patch(this.port2);
  }
}

where patch wraps onmessage/addEventListener to get a registration-time snapshot, or as an alternative to store the .postMessage-time snapshot on this.__channel and run the event callback in it.