My Mac Pro 1,1 won't boot from any type of usb stick
I think whichever way you force the newer firmware, you're going to have to have the installer on HD or DVD. I've never been able to boot a pre-09 Mac from USB &amp; I've tried pretty much everything the interwebz has to offer in that regard - though my oldest Mac Pro was an Early 08 so I've never needed to force it quite so hard, as I don't need to push the firmware upgrade too. 

Here is a guide on how to get Yosemite onto a 1,1 They reckon it can be done from USB, but I'd say YMMV &amp; be prepared to have to run it from a spare hard drive instead.
I'm not certain if the investment in a newer Graphics card is compulsory for functionality, or whether it's just to get the machine to be as fast as possible after the upgrade, same as the CPU swap at the end.
I've an old GT 120 I have knocking around that I occasionally use for troubleshooting &amp; it has no problems on 3,1 4,1 or 5,1 machines.

The longevity of this particular method will depend on how long mega.nz has the patched files, I guess - &amp; also whether you have any particular reason to trust or distrust a non-Apple source. On the latter I really cannot comment, though I can see no reason anyone would go to all this effort in order to be able to botnet the few remaining Mac Pro 1,1s out there ;-)

http://www.pro-tools-expert.com/home-page/2015/3/2/how-to-resurrect-a-2006-mac-pro-11-so-it-can-run-osx-yosemit.html

Precis:-  


  Things that you will need:
  
  
  A Mac Pro 1,1 or 2,1 Cheese grater desktop with OSX Lion 10.7.5 system drive
  8GB USB drive
  A graphics card better than a GT120, I used a Radeon HD 5770, original mac version with its appropriate power cable. There are flashed PC cards out there, but they won’t display a boot screen. I like a boot screen, so I purchased a proper Apple card (2nd hand)
  A download of a pre patched version of OSX Yosemite 10.10 https://mega.co.nz/#!SoAgnKqI!GUinePUGF3jAAu3oUym-GHBC9doWRTU1p6nnkXkJFdQ
  Either boot.efi file downloadable here. Black is a white Apple on a black background and Grey is the traditional Apple boot screen:
  Black:      https://mega.co.nz/#!G9oimYaK!38dBEPUwPb2y8Inh0w-REL6MH-9K90vZjpMSzazNy-E
  Grey:       https://mega.co.nz/#!j1hWFSIS!4zFpN9f3L6zV_D5wXJ1KMT40KkLlXKQ901-eQaRaLTA
  I chose the black screen as I dual boot into OSX Lion 10.8.5 or Yosemite 10.10.2. The black screen tells me instantly that I’m booting into Yosemite.
  A spare hard drive, mechanical or SSD for the new system drive. You could partition your old Lion drive, but I prefer to build on drive that’s minty fresh. If I stuff anything up, I can always boot back to my untouched Lion system drive. Contingency plan…
  A download of Yosemite 10.10.1 or 10.10.2 Combo updaters http://support.apple.com/kb/dl1786?locale=en_US
  [That would now be 10.10.5 - see http://www.apple.com/us/search/combo-update?src=globalnav_support]
  A download of Diskmaker X http://liondiskmaker.com/
  
  
  Open up Lion Diskmaker X and follow the instructions to create a bootable USB “special” Yosemite installer from the big Yosemite image you download before. This is the patched version of Yosemite.
  
  Don’t install the Apple App store version as this does not have ze patch…
  
  http://www.cultofmac.com/313198/how-to-make-a-bootable-os-x-yosemite-drive/
  
  
  Install Yosemite from USB drive to your new “Yosemite HD” system drive. Bay 4, remember?? Hold down the Mac option key to choose your boot drive after start up chime.
  Install the boot.efi patch, I used the black version. Restart the mac, hold down the mac option key to choose your boot drive after start up chime You should now be able to boot into either OSX Lion or Yosemite 10.10 flat. Woo Hoo!
  Install the Combo updater
  


Edit
All the mega links are already gone, I'm hunting for replacements &amp; will post as I find them.
I've asked a dev friend of mine who has a post-Lion 1,1 if she has a source. Will add to info if we can findone]

New bootloader here - https://github.com/Piker-Alpha/macosxbootloader