For any CfgVehicle entry in a config, you can add eventhandlers. Using the init eventhandler, the script will execute (locally of course) for each unit as soon as it is created (or at the start of the mission). It looks like this:
      class Eventhandlers
      {
         init="_this exec {\path\script.sqs}";
      };
The path will allways start with a "\" if it is inside of a .pbo file. The rest of the path is simple, just treat the .pbo file as a folder.