[Gandalf Parker]

This isnt the answer you want but some tips to tide you over for now...

There is a difference between orders that are given to troops vs orders of their commanders. The basic troops are a bit.. simple minded. In order to get better results, use the commanders.

Attack rearward is decided a lot by what they can "see" as rearward when they begin the action. Setting them to the flanks helps. Telling them to hold and THEN attack rearward helps more. Having the bodyguard a commander who is told to hold more than two turns then THEN attack rearward can help even more. Or have the commander attack one round then attack rearward. Or a combination of hold, hold, attack one round, then attack rear.

Commander archers can do the above with orders to fire at rearward. One good commander with a special bow can do alot of damage. The Bow of War fires a volley of arrows all by itself.

For important battles it might be beneficial to use "fire then retreat" in the scripting. They will have to be brought back up to the battle but it can give you the "dont fire at routing troops" option.