So if I understand that correctly, I want to create a trigger zone over the object I want to destroy (building, bridge, etc) and create the script to look like: 
 mist.flagFunc.mapobjs_dead_zones{  
  zones = {'bridge1'},  
  flag = 10
}     
Where the bridge is within the trigger zone "bridge1" and when destroyed Flag 10 will = True?   
I need to really read up on MIST and how to use it.  My brain struggles with programming language, but its something that I'm going to need to learn if I want to make more dynamic and interesting missions for me and my friends to fly...