I know they patched the GPD modding LONG ago, but I have another theory that may lead to mods back online. I'm not sure if this will work or not, but considering the TU's are downloaded and saved as an actual item on the HDD, you're free to move them to a USB or Mem card.

Think of it this way, if you tried to play modded GPD zombies online now, the mods won't work. Why? The TU is automatically loaded. The way I picture it, is if you mod the account and then move it to your HDD, then take the TU and put that on your USB and then take the USB out of the xbox. Once done so, boot up BO and sit at the start menu. Plug in your USB, push start, and select the HDD. The TU should automatically be loaded, even from the USB. Once you load it, and you can see the downloaded maps you have in zombies, start up a game on any downloaded map you want. Once in the game, remove the USB.

I have not tested this method as I can no longer connect to XBL. I have not attempted to try this with an offline version with the TU but will soon. Anyone that can test this and report back to me if it worked, I will give rep to. If this method works, it CANNOT be patched. Why? The TU can't affect the game if it's not on the console.