This is getting pretty specific. I would recommend reaching out to your Account Manager with a URL where we can see your set up.
The tag will fire on DOM ready but it will load earlier on page load. So depending on where you are loading utag.js and when the iframe loads the above methods may still work without problem.
Also, if you're using legacy GA that method will still work. You just need to change it to:
_gaq = _gaq || ;
_gaq.push(['_setReferrerOverride', 'NEW URL TO SET AS REFERRER']);
Then when the tag does load it will just use the "_gaq" array that you previously set.
Our team will be able to look deeper if we had a URL or an example of how your page is set up.