I've just managed to install this from the stock 4.1.2
It was a little tricky, but it is possible.
1. Download the AIO Tool from this thread
http://xdaforums.com/showthread.php?t=1468298
2. Download the update package from the first post of this thread. Put the zip file on your SD Card.
3. Follow the instructions in the AIO thread to make sure your bootloader is unlocked.
4. You now need to install the CWM. This is the tricky bit. What worked for me was:
a. Unzip the contents of the LAIOT zip file to your hard disk.
b. Go to the folder LAIOT/Recovery
c. Right-click and hold the shift key down in the directory. Choose '
Open Command Window here'
d. Make sure your Xoom is on and USB debugging in enabled. connect the Xoom to your PC with a USB cable.
e. In the DOS window type :
adb reboot bootloader
f. Your Xoom should now reboot and say something on the top line about Fastboot mode
g. Still in the DOS Window type :
fastboot flash recovery CWMTiamat.img
h. You should now see it flash the recovery and say success
i. Still in the DOS window type :
fastboot reboot
j. When the Motorola Logo appears press the
volume down key, then press the
volume up key. You should now be in recovery and able to follow the instruction in this original post from Point 4 in Procedure and flash the zip file.
That's it, it worked for me. The only thing I found was that if I did not immediately go into recovery after point i. and j. above I has to start the procedure again form point e. above
After following the above completely and flashing the zip and after a reboot it is possible to go into recovery using j.
Hope this makes sense. It did work for me.