Observers

Can I enable my map to have observers (idle players) without using first 12 player slots?

How does the blizzard observer mechanism work?
I noticed there are some functions and variables for this so there must be some built in support...

No, you can't.

You can detect which players are observers, though - it's a player state.

you may force players to be observers after game started.

its integer player state (0 and 1)