Jonathan, this file is dynamically generated html page. It may be stored somewhere, but it's not clear if it is accessible to be changed. You may want to consider creating a virtual proxy using ticketing and with that you can create your own login page to authenticate to an identity verification solution of your choice.
More about the ticket api here: Qlik Sense Proxy Service API ‒ Qlik Sense
and examples abound on branch.qlik.com
