Even before it was available to general public, pro scene used to be played on Hexakill. As spectator didn't exist back then, the casters would load in as the sixth champion on both teams to observe the game. They would normally remain in fountains except for that one time when a caster (Phreak I think) accidently moved his Teemo into lane.
^
Very first version the spectator champs they were using as a workaround for broadcast were targetable, from memory one got killed by a Karthus ult giving extra gold to one of the teams even. Version after that had them untargetable and invulnerable. Then we got the actual proper spectator system in.