Yeah, you can flash it with Xellous, I had to do that when my first upgrade attempt didn't work.
Assuming you made a backup of your XBR3 nand before you started messing with it (and if you didn't, good luck to you sir) make a copy of it, rename it updflash.bin and follow these instructions from the link you found
Copy updflash.bin to the root of usb drive formatted as Fat/Fat32.
Ensure XBOX is off, Plug usbdrive and turn the xbox on so that it boots into XELLOUS and watch it flash your nand.
Follow the on screen instructions. When power cycling ensure XBOX is unplug and off for 30 seconds.
Please beware that a dvd drive needs to be plug in for booting into XELL, at lease the small black power cable.
However SATA from dvd drive does not need to be.
from XELLOUS release
For best results of getting the usb device detected. Remove the power plug from the console
after running the MS dashboard. Then reinsert the power plug, insert usb device and then
boot into XeLLous.