I know this is a very late reply, but in case someone ends up searching for this and ends up here.

The way of having an iframe inside a UI Page is currently very simple

<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">
	 <iframe src="some_url_source_here" width="400" height="400"></iframe> 
</j:jelly>