This sounds cool. And even if it doesn't work out it sure will be fun to watch. I can only imagine the fun you could have with that many people
Ideas:
You could line the teams up, give them sniper rifles, and have a kickass Napoleonic style rifle battles.
Use teamchat and start your own Choir
Make a giant human pyramid
why this won't work:
Getting 100+ people to work together would be impossible (it would be awesome if you did it). Getting internet people to be a team is like herding cats
Game modes like CTF will have to be rethought as the flag holder can have a human shield 20 people deep
What if one team all had grenades or rocket launchers?
Anyway it will be fun to watch