Okay... so I was feeling saucy tonight and I did it!
I essentially cloned my main Lion boot drive to another drive in my MacPro1,1. Then, removed the drive, put it on a sled that would fit my MacPro4,1 (they're different) and then booted from off a Snow Leopard partition containing the ML installer as well as the modified "boot.efi" file. (It would not let me run the ML installer booted from Yosemite.)
I then essentially "upgraded" my Lion drive... I did NOT reformat first or anything like that. Once the install was finished, and it restarted, I held down the Option key, chose the Snow Leopard volume again and then followed the instructions to replace the old "boot.efi" file with the new one using Terminal. It got a little hairy as the "boot.efi" file in the "... coreservices" folder did NOT want to be replaced or deleted. I finally figured out how to remove the "locked" status in Terminal and then was able to drag the new "boot.efi" over in the finder and it finally allowed me to replace it. I then immediately shut down, took the drive out, put it back on the sled for the MacPro1,1, popped it in and then booted with the Option key down just to be SURE it booted from the ML drive the first time.
It booted in verbose mode which was a little scary, but I was patient and then it came up! Mountain Lion! First thing that hit me was something barked at me to install Java which I did, and then Spotlight started indexing which made it unresponsive for a bit... but now! Hey... it's working. Haven't tried messaging yet (which is what I really want... but we'll see.)