Unfortunately not. Qlik Sense Authentication for default proxy is a compiled .exe file with data
and css encoded inside.
You could use a reverse proxy and rewrite some part of the login page, but if you already setup a
reverse proxy, you are already very close to getting your own authentication page.
You can change the hub, do everything in the mashup. But the login page can't be changed.
Someone said that there is also a license clause that prohibits to change the Qlik login page.
By the way, the Qlik authentication is also faulty: sometimes you get stuck with a white page, issues with refresh/reload and the cache of the browser. So, I suggest developing your own anyway.