What if u set a trigger activated when '_convoy KnowsAbout _assailant > 1'?
With this condition (or a 'Detected by convoy's side' type trigger) the convoy's soldier will notice that somebody's approaching... You could have a 'UnassignVehicle' command for each soldiers u wanna have out from the trucks. If u don't want ur soldiers to engage at their will, set their combat mode to HOLD FIRE (_unitname setcombatmode "green") and they'll fire only to protect their selves...
I hope it works...!