Cube2, another oldie but goodie FOSS games relies heavily on admins too. They are supported with stats (a server plugin, IIRC) and a observer mode which is actually available to all spectators (the game is more focused on fun than competition).
Cube1 solved the problem by keeping the communication stack closed source so you couldn't build a aerver-compatible client. The OSS version of Cube1 used a different communication stack.
