Hey Matt -
Offically? No. You can't do this.
That being said, the Tableau Server portal is built with Ruby on Rails (among other things). Unofficially, all you'd need to do is find the ,erb file which is leveraged when the authorization page is shown and modify it.
Doing so is unsupported, of course - and any hacks you make will be overwritten when you upgrade. If you're interested in trying anyway, I'd suggest you focus on finding *.erb files which mention "login" somewhere in their name.
Don't mean to be obtuse on this stuff, but you'll need to figure it out on your own and understand that there is risk involved whenever you start playing where you're not really supposed to