From my experience doing a zombie game night awhile back, I'll shed some of what my experience was and the issues.
1. Hosts vs Participants - People love zombies, but there is a limited amount of space in each lobby. Significantly less than in normal game modes. Which means you may have to deny people positions (which puts people off when it comes to future events).
2. Game Time - Zombies is a continuous, nonstop game mode. There are no breaks or chances for people to go away if they need to. Normally this period would be used to bring in people that are waiting. With no breaks in game play there is no chance to bring in people.
3. Organization - This ties in with the game time. Once you are in a game, how do you plan to organize the rest of the people that show up to be sure that everyone that shows up gets to play? Remember, you are the hosts and are in charge of the organization, if you're in a game you wont be able to coordinate between your game and the other lobbies that you would need to be setting up.
Zombies is great and all, but it isn't made for large groups. The purpose of these events is to bring the entire community together to play. The limited space/constrictions make this a more than daunting task to say the least. I'm not saying that it isn't possible to be done, just that it is way more of a headache/challenge than it looks on paper. I would like to see how you plan to run/organize the entire event before you attempt it.