Just so that we're clear on my use case here-

For my internal ZenDesk all of my users only use Google SSO to login. The separate username/password fields are incredibly confusing and generate multiple calls a week of "I can't submit a ticket". Having to roll an entire custom SSO solution is entirely unacceptable just to change the look of the login page. There are other reasons I need the branding here, but this is the most pressing. Had I known we couldn't do this I likely would have gone in a direction other than Zendesk.

I love ZenDesk for almost everything that we do, it's a great solution, but clearly I'm not the only one that is both frustrated with the service being hamstrung by this one feature and the uncommunicative, totally opaque nature of the updates we are receiving.