I can imagine that it'll be fun just floating around with your buddies acting like drunkards, but I guess that spectators are invisible for a reason, probably because the Devs don't want to go into the trouble of coding something like that as how I see it, it looks like something difficult to pull off. But then again, I have no backup, experience or reason to show why so, if it is easy to do the biggest problem would be the spectator model, which shouldn't be an issue, just make a white sphere and a black hole in the middle.
I also believe that no model for a spectator was made because the Devs wanted Tremulous to make sense, if floating eyeballs were going crazy around the field, it'll really put off the serious yet Sci-fi-ish tone for Tremulous into something more comedic. Maybe a floating watch bot, like in Deus Ex:IW or HL2, but problem is it'll take some modeling time into something that isn't really necessary to do.
Btw, I believe this is best put somewhere in Feedback or Mod Ideas, not here. This section of the forums is used to help other members deal with trouble coding, modeling or etc because your giving us something like a suggestion. (Or are you trying to actually do this but asking how to do it properly?)