Just a clarification to the OP: Everything in the Mac Pro 1,1/2,1 is fully 64-bit except for the EFI. The EFI used in these machines is EFI32. That is why you cannot load 64-bit mode without Chameleon or other bootloader. This is the part of the system Apple would have to update for us.
I'd love to see it happen since it would make running ML/Mavericks that much easier (one would only have to add the MP1,1/2,1 board IDs into a couple files in the OS proper), but fat chance of that happening.
The Xeons used in the 1,1/2,1 MPs are 64-bit and based off of the Core 2 Duo architecture, not to be confused with the Core Duo architecture, which is 32-bit.
The iPhone 5S is the first to use a 64-bit CPU, but until that is realized by software developers, it's merely a marketing ploy.