The only Modifiable thing on OFW for Black ops 2 is the first level of the campaign. Since the GPD can be modified offline to give you some simple cheats in the campaign.
The only other way to "mod" black ops 2 on OFW is to have someone else running a Jailbreak load a GSC menu then give you access in game to it via the permissions that most menus of that nature have. It only works in the games that modder your with is hosting and is limited to what the modder gives you permission to use. You will often have to pay a sum of money to even get a modder to bother letting you use the most basic functions in any match. Some might even freeze your PS3 instead of giveing you access.
Anybody saying they can give you Multiplayer mods or Zombie mods that you can host your self on OFW is scamming you.