Along the OPs desire to award the active participants for winning war effort, make the reward something that has value to be used immediately. Let's say, half priced sale on perked rides for 1 hour upon the map reset. "The spoils of the war."
Could be a ton of other expiration time ideas, the above is just a sample. The idea is to promote folks to "actively" participate. If you were sitting in the tower asleep when the war was won, and your reward expired before you woke, oh well.