Scripts

In principle, your approach is correct. Usually (in campaigns), the armies/garrisons/castles/caravans that you want to make inaccessible for attack are given to an allied player.
I use the following method in scenarios: I create players:
a) us
b) n enemies
c) a player loyal to us
d) a player not loyal to us
Essentially, there's nothing wrong with adding an allied player. And no workarounds are needed, although I have one: simply check in the processing loop how close we are to the caravan, and if we are too close, stop the movement.