I am not sure if this could be done, most probably not. What comes into my mind in this case is to try showing RadAjaxLoadingPanel from the page loaded in the iframe, when it is initially loaded as here.
Thanks for the link. One possible solution I'm looking into is using a RadWindow instead of an iframe and embedding it into page since this has a way to use a RadAjaxLoadingPanel with it.
The whole point behind this is to allow me to nest a Google Map control inside a RadAjaxPanel. Since it doesn't play nicely with ajax, i found this to be somewhat of a decent workaround.