EDIT: This doesn't work anymore.
This is due to the same-origin policy. When an iframe gets accessed by the root origin (your website) it seems to also change the origin of the iframe. So the video can't load from a different origin (youtube.com). See my test on JSFiddle.
I think the fact, that it worked before was a XSS security issue which has been fixed recently. So I can't imagine modifying something in the youtube iframe is even possible anymore. At least not in this way.
Thanks @maxple for pointing out!
Original post:
This should be possible with newer Browsers and the HTML5 Iframe Sandbox Attribute:
With the option you can access the iframe DOM node.
<iframe id="myframe" sandbox="allow-scripts" src="about:blank"> </iframe>
<script>
var frame = document.getElementById("myframe");
var fdoc = frame.contentDocument;
fdoc.getElementsByTagName("video")[0].playbackRate = 3; // or whatever
</script>
See this post for more info.