The reasons why loaders go in position as soon as you start a service, is exactly to PREVENT what you are describing, because with the loaders already at the doors, they are at least in a predictable position, while if they were still in their startup position when the catering arrives, the risk of the catering driving through them would be way more likely, since the startup position is always different, depending on the parking size.
This means, BECAUSE they go immediately to the door, it's very unlikely to have the catering truck drive through them, except in the most cramped parking spaces, or with airplanes where the service door is very close to the front cargo door.
If the problem is the scenery parking position to small, the solution is to use the GSX Scenery Editor and just disable the front baggage loaders (GSX didn't have any before version 1.3) on THAT parking position.
If, instead, the problem is the airplane having the front service door too close to the front cargo door, you can use the GSX Airplane configuration editor to disable the front loaders for that airplane.
