run a script that makews a boolean come true, then have the trigger activated at that boolean or simply have the script wait until the boolean is true
init.sqs entry
rescue = false; publicvariable "rescue"
script entry when the soldier has been captured
rescue = true; publicvariable "rescue"
and then the line that waits for rescue to have occured
@ rescue
you could probably simply place in the condition field of the trigger
rescue