I successfully updated my XBR3 Xenon 1921CB0 Box to Freeboot with this.
I used Win 7 x64 and a current XBR3 dump from my console (aquired with Flash360).
Small Tutorial:
Dump your current NAND with Flash360 on a USB Stick.
You can aquire your CPU Key and CB0 info with 360 Flash Tool by modfreakz. Just open your
flashdump.bin you got from Flash360 in 360 Flash Tool. No need for a hexeditor. Write all that down
and keep it somewhere safe or at least keep several backups of flashdump.bin.
Once you have your flashdump.bin loaded in the app and your CPUkey, Boardversion (Xenon, Jasper, etc) + CB0 correctly selected, hit the buttons from top to bottom.
After hitting the last button a folder window opens. There you select the updflash.bin and copy that to your usbstick where you also have Flash360.
Now, before you do anything, I'd advise you to compare your flashdump.bin (thats your XBR3 dump) with your updflash.bin. To do this, open both in 360 Flash Tool and look in the columns.
The left column (Bootloader) should be the same until "6BL". Your updflash should say 9199 there.
The right column (keyvault) MUST BE exactly the same for both dumps. If 360 Flash Tool reports any errors in the keyvault column for your updflash.bin DO NOT FLASH THE IMAGE ON YOUR CONSOLE.
After you checked your updflash.bin stick the USB Stick into the console, boot it, start Flash360 from Xexmenu.
Then select to update your onboard NAND (Button A).
Then select to flash the entire NAND including the config area (Button
[this is why you double checked your updflash.bin before]
If Flash360 throws up any error messages after pressing B, be safe and don't flash but rather go back to your PC and recheck everything and redo the whole procedure.
Normally Flash360 will just compare the keyvaults and ask you to start the flashing procedure by pushing A.
After you started the flash procedure, do not do anything with your console or with your controller that could interrupt the flasher.
Once it is done, press any button, press BACK and press BACK again to shutdown the console automatically.
After the console powered itself off, remove the Powerplug from the wall socket for a minute.
The reboot your console, if all went well you should be greeted by the initial setup screen.
Good Luck.