[StevenB]

• Definitely allowed. But as others have said, talk with the regional coordinators, since they're the ones who will give you permission to set up next to the field and tap of the big screen display. They're also the ones who can give you a fast wired internet connection.
• Equipment: The easiest solution is to get a USB composite video dongle, such as this one (I've used this super-cheap one successfully on Linux, but YMMV on Windows 7/8). Then you just need a laptop that's at least kind of modern. My 7-year old machine struggles a bit, but anything newer than that should be fine.
  
  For better quality, you'll need an HDMI video capture device, which tend to cost about $200. I've used a Black Magic Design Intensity Pro with good results. You'll also need a much more powerful PC; I had an 8-core AMD something-something from a couple years ago and it was churning pretty hard.
  
  Talk to the AV people, I've usually found they're not talkative and are kind of doing their own thing, but are friendly enough and are happy to help give you the feed in the format you need.
  
• Twitch setup: It's quite easy - less than an hour of setup if you've never done it before (and aren't too unlucky). Basically you sign up, find your "stream key" from your profile, and then configure OBS or whatever software you're using (I've used ffmpeg/avconv) to broadcast with that stream key.