If you have access to another Mac that is Yosemite "legit" get ahold of an SSD or any 2.5" drive and a $5 USB adapter. Run the install on the other Mac, then migrate anything you need from that Mac. Restart from other drive on the other Mac and place the boot.efi files, repair permissions and YOU ARE DONE.
Move the drive to unsupported Mac and enjoy! If you want or need it on 3.5" drive use Restore.
It really is a whole lot easier than hacking the installer. I have 2 unsupported Macs running via this method. Worked the first time and I moved on to other things. If it has become a major endeavor you are doing it wrong.
Good advice, after some struggle with USB-installers I choose this method and it worked (Mac Pro 1,1). So much easier this way.
I also did it like that for my white MacBook 4,1, it has 64-bit EFI but different challenges. Didn't work at first, then I noticed for some reason it was booting from the recovery partition so I had to modify the supported list to that one too.
Why this model isn't supported is the graphics. Found drivers for audio and GMA X3100, the machine is pretty usable with them but no OpenGL. And I don't do much with a machne without it. Maybe should try Mountain Lion with 32-bit kernel.
But I'm happy for the Mac Pro, will make some donation for the Pike guy!