Actually the short answer is yes. Well, the long answer is yes anyway. olseric is tecnically correct, there is no direct way of doing it. However, as so often in OFP, there is a workaround.
The game fills the cargo seats in a specific order. If you want the player to be in particular seat then you must fill all the preceding seats. If you want these seats to have loons in them that's easy, just put them in first. If you want them to be empty put a gamelogic in them. Or a mixture of both. Treat GLs like ordinary loons in this case: create them somewhere on the map, name them, and use moveInCargo commands. GLs are
You cannot make him sit where there isn't a seat unless, as olseric says, you alter the model.