One problem with this might be server scripts giving them a player object, which would break security.
I'm not 100% sure, but I think you can download add-ons in the demo mode.I believe the only restriction of the demo mode is the multiplayer, and ofcourse the brick limit.
What about having them as cameras like F8 only, with no chat or player. They can see people's chat but not interact with the server at all.One problem with this might be server scripts giving them a player object, which would break security.