It's actually easier with your RX 480, since it already has it's device ID in the framebuffer kext, thus it'll load, just without acceleration. A non-EFI Maxwell GPU won't give anything but a black screen in OS X until the drivers are installed, so you need a 2nd GPU or install the drivers through VNC.
At least Apples drivers don't crash any longer with a Maxwell GPU installed, they just won't do anything.