Perhaps prompting the players to accept each change (and requiring everyone to do so) whenever another player sets a multiplier and then hits "ready".
So if a group of players is in agreement that player A can have a resource multiplier of 2, then player A would set his resource multiplier at 2 and hit "ready". Upon doing so, he's in "limbo" until the other players either click yes or no in a popup prompt. If everyone clicks yes, the player remains "ready" and keeps their resource multiplier. If anyone clicks no, then the player will become "not ready" and the resource multiplier will return to the default.
But at the most basic level, it'd be a shame to exclude such a feature just because of fears of players abusing it.