I have another idea, but I don't know if it is possible. I did some tests with the hydra shooting at some barrels and it worked. The only problem is, that if the player starting in a hydra is a cheater, he could blow up all other players.
My solution is, that all players start in fire trucks, except for one. (Just by placing a hydra vehicle change pickup in one spawnpoint.)
If it is possible to tell the game that the owner of the server (or someone with a particular IP, so the owner of the server could change that to his own IP) starts in the spawnpoint with the hydra pickup, and the players with other IP's will start in the other spawnpoints, the problem is solved.
But is it possible to tell the game which spawnpoint is meant for what IP?