See https://rawgithub.com/omo/8574605/raw/128c8768a368017a1adf0446d645e33da780f31e/index.html

decorated-body.html

<!DOCTYPE>
<script>
window.Document.prototype.hello = function() {
  alert("Hello!");
}
</script>
<h1>Call document.hello() for me.</h1>

index.html

<script>
function test()
{
    window.myFrame.document.hello();
}
</script>
<iframe name="myFrame" src="https://rawgithub.com/omo/8574605/raw/94552e0669a6dec3dab7485109691cd158bacba8/decorated-body.html">
</iframe>
<input type="button" onclick="test()" value="Press Me!">