Just some more peanuts from the gallery.
The Mac Pro 1,1 for all purposes is capable for running a 64 bit operating system. There seems to be confusion here.
Are there operating systems that run on the 1,1 MP that are 64 bit? Yes
OSX and MS Windows and Linux.
Are there limitations to the 1,1 MP - YES
The 1,1 uses hardware designed for mother board bus which is limited to PCI 1.2. Later MP's run on a different PCI bus - 2.0.
Later hardware and operating systems may or may not be backward compatible. For the MP most hardware that is meant for PCI 2.0 or later will not run on the earlier Mac Pro systems. Case and point - various video cards.
Is this limitation addressable - in most cases YES. Does Apple or 3rd party have an obligation to make these items backwards compatible - NO.
As someone who owned a Mac Pro 1,1 I sure felt screwed but then again, I also know that lots of newer software and hardware would not run on any of my older PC's though they were capable of handling and addressing 16, 32 and 64 bit operating systems.
As for the EFI - I have used in the past DOS based PC's meant to run 16 bit DOS and yet, was able to run 32 bit Farlap extended DOS that ran 32 bit programs. Also it is not a secret that early 32 bit Windows ran on top of DOS that was 16 bit. The hardware handled this fine and was a "fully functional 32 bit" system.
My Mac Pro 1,1 was maxed out when I sold it. It was a fully functional 64 bit systems that ran a PCI 1.2 bus, with PCI cards that worked on that particular bus and an OS that supported PCI 1.2.
As for the EFI, there is no reason that OSX should move forward and add an abstract layer to handle older designed EFI. It is up to the manufacturer to decide. Apple could at any time elect NOT to support other operating systems as well and yet remain a "fully functional 64 bit OS."
Apple only need demonstrate that specific operating systems run on their machine to show it is a "fully functional 64 bit" computer. Apple doesn't have to show it handles multiple operating systems or upgrades to operating systems.
Like others, I felt a bit cheated when newer stuff came out and my Mac Pro 1,1 was not fully able to use newer devices/hardware and later operating systems. - It had clear limitations with the biggest being PCI 1.2 bus standard.