Well yes, the access exception is due to the fact that document.domain
must match in your parent and your iframe, and before they do, you won't be able to programmatically set the document.domain
property of your iframe.
I think your best option here is to point the page to a template of your own:
iframe.src = '/myiframe.htm#' + document.domain;
And in myiframe.htm:
document.domain = location.hash.substring(1);