Since it was explained pretty clearly in the other thread I will summarize for you.
1) Make an assist macro.
2a) If you want to stay completely legal, script one of your buttons to sent just the key you bound to assist on all of the other instances.
2b) Or if you want to risk getting banned create a script that sends out the assist, waits depending on how long assist lag is in WoW, and then does the action you want all instances to do.
You cannot "Automate" with HKN since it does not create for/while loops(nor should it). HKN also does not have access to WoW states(out of combat, in combat, etc) nor should it. The best(and imo it is pretty damn good) you can do is set it up to make all your toons manually assist at the same time when you tell them to.
The closest you are going to get to "Automatic" targeting is if you have HKN capture the key(can't use mouse buttons yet iirc) you use to target on your main, and then sends that key to your main and the assist key to all your other instances. Assuming the assist key is different than your targeting key then you cannot use broadcast key but will have to explicitly state the hotkey.