You can do this using init.sqs and game logic.
Create a script in the mission folder named init.sqs then open it with notepad or another text editor, i suggest ArmaEdit. Then place a "game logic" unit wherever you want on the map and give it a name (I'm using vehicleLogic for the example) and the vehicle you're gonna use (I'm using cargoVehicle in the example). Give a name also to the group leader (I'm using cargoLeader in the example) you want to start in the vehicle. Then write the following code in the script file:
vehicleLogic moveInCargo cargoVehicle
~0.01
{_x moveInCargo cargoVehicle} forEach units group cargoLeader
exit
EXPLANATION:
The game logic is an invisible unit with no AI, but it is still a unit so you can make it board vehicles like any other unit. The result is that if you make it board the vehicle before any other soldier there will be an invisible unit placed in the co-pilot seat and thus the units will all start in the back and they will see an empty co-pilot seat (same for trucks, with the difference that, if I remember well, in OFP there are 2 seats next to the driver seat so you'll have to use 2 different game logic and make them board the truck before the units).
The first command make the game logic start in the cargo space of the vehicle (the first available is the co-pilot seat), then waits for 0.01 seconds and after that make every unit in the group of the cargo leader get in the truck. Every unit in that group will start in the back seats because the seat unit is occupied by the game logic. Remeber that if you want to be able to use the co-pilot seat again you'll have to delete or move away the game logic (for deletion use the command deleteVehicle, for moving use setPos). I hope 've been clear enough.
EDIT: I don't know if it works, but instead of writing the script, you can try to write the line
this moveInCargo cargoVehicle
in the game logic init field, and then writing
{_x moveInCargo cargoVehicle} forEach units group this
but I'm not sure it will work since the order in which the init fields are executed is not predictable, so it might happen that the init field of the commander is executed before the game logic one, so you'll have a vacant seat in the back instead of the co-pilot one. If you use the script you're sure the game logic will be placed correctly, but you can still try the second method.
EDIT2:
I misunderstood your request... Well I thaught you how to make a vacant seat and it could be useful
!
If you want to make another pilot board the co-pilot seat then simply replace the game logic with a pilot unit, then the script it is the same. Use the script for the same reason explained in EDIT1, since the order of execution of the init fields is not predictable, and that's probably why the pilot was alwasy seated in the back.