Ok so after reading around and doing some of my own playing around, I figured out how to fix it with some ideas from this thread
First off, the moto x rooting process actually REMOVES the recovery.img file from your phone, so what seems to be happening is that the upgrade downloads and starts to install... it attempts to load the recovery to flash the image, but since it doesn't exist, it just restarts the phone. When the phone restarts, the upgrade process still thinks it needs to run the upgrade, so it powers down.. this is where it gets stuck in the loop.
The solution is actually simple. Get into Bootloader mode (Power + Vol Down) which was slightly tricky since the phone always rebooted and never actually powered down. So I had to catch it at the end of the power off cycle, before it restarted. Once I was in bootloader mode I could use the following command:
fastboot erase cache
That seems to remove the downloaded update files as the phone stopped rebooting over and over.
But now I get Mobile IP Registration Error 67 (MIP) which basically means my registration data got wiped.. not sure I understand why that would happen.. but it basically means I need my MSL and have to enter the code to reset registration which will force a wipe of all data. At least I have the opportunity to back everything up over wifi now that I've stopped the looping.
The second part of this fix I haven't tried, but based on an educated guess, to actually GET the update correctly, I would need to flash the stock recovery image back using:
fastboot flash recovery recovery.img
where the recovery.img file comes from the zip file for the moto x stock image from the sbf repo:
That of course needs to be done before I call Sprint to get my MSL and perform the reset.
All in all, a minor setback overall... better than a paperweight
Perhaps instead of doing the erase cache step and just flashing recovery may have saved my MIP data
Ah, gokart2, I see you had a similar process. Cheers