To avoid prearranged draws, consider structuring a tournament with random pairings each round and players sequestered from each other so they cannot see whom they are playing. How difficult would it be for players still to communicate with each other, confirming each other's identity then agreeing to a draw, through their moves, perhaps opening moves?
Do things become easier or harder if players are permitted to see the other games in progress, without the players' labeled of course? On one hand, it might make it possible for a player to determine who other players are by observing style and opening selection, and determine who the player is playing by a process of elimination. On the other hand, visible games could help a player pretend to be another player trying to take advantage of draw prearrangements between other players. Maybe the visible other games are delayed.
Chess960 would probably make it more difficult to communicate via opening choice.
Having a few weak players in the tournament might decrease the tendency of prearranged or short draws. The strong players cannot know for sure whether they are playing a weak player against whom it would be a shame to give up a draw.