Editing Pro Sites sign up form

I have a multisite where Pro Sites is used for all new site registrations. "Signup on Checkout" is enabled at Pro Sites's settings and Pro Sites is rebranded to "Premium".

So now mysite.com/wp-signup.php redirects to:
mysite.com/premium/?action=new_blog

Problem: I want to customize texts and input fields from the registration form that shows up when user selects a site level, or signs up for a free site.

I assume that Pro Sites uses WP's "core wp-signup.php"-file for sign ups?

I know it's possible to change wp-signup.php location:
https://premium.wpmudev.org/forums/topic/custom-wp-signupphp#post-301498
(And 1000 other topics)
(or using anti splog and custom-wpsignup.php)

But with Pro Sites and rebranding (from PS's settings) this seems impossible.