PostMessage into a dynamically created iframe.

gistfile1.js

var frame = document.createElement("iframe");
frame.src="https://<the mothership's domain>/inner.html";

frame.onload = function() {
  // If this is a Chrome content script, contentWindow is offlimits.
  frame.contentWindow.postMessage("A Message!", "https://<the mothership's domain>");
}

document.body.appendChild(frame);