First just let me say I\'m open to ideas on a different approach altogether.
I have and iframe as such:
Add this to the -elememt:
onload="$('body>*',this.contentWindow.document).not('#ctl00_CLPMainContent_Login1').hide();"
it will hide every child of the body except #ctl00_CLPMainContent_Login1
If #ctl00_CLPMainContent_Login1 contains more than the loginbox, you have to use the suggestion using clone() posted by pex.