1. Rip GoW3 to your JTag HDD
2. Download link at the top and then replace the files in your GoW3 folder on JTag
3. Play KOTH until you are rank 100
4. Then go back on your retail xbox and play a few games online to make sure it sticks
Credit goes to SHAWNJOHN1120 at 7SINS for figuring this out but credit to me for making the tutorial easier to follow.
Apparently they also have figured out how to unlock the DEV character skins and weapon skins and get them to carry over but wont release it.