Honestly? You'd be much better off putting the plugin in a div at 100%x100% and then managing the border of the div. For something like this I'd probably just use:
m_host->evaluateJavascript("document.getElementById('pluginCont').style.border = '1px solid black';");
That'll be the easiest. You could also look at the DOM abstraction code and add some tools for managing CSS; note that on IE you may need to use special activex methods to do this, which is why I dont' recommend just doing it through getDOMElement()
(which is a shorthand, btw, for the long code you have in your example)