Defending Alliance Members!!!
Actually it should be easy to apply this to the alliance as only the alliance members that are on line could help anyway (same as an alliance member or friend can send res to you while your being attacked) you should be able to implement the option of sending troops to an alliance member or (friend).
Enemy reinforcements usually don't attack until you've been in the siege for what 19 minutes or so. It would still be that same time period for your alliance member/friend to come to your aid. It could still be on the same toggle method you mentioned in the other article. You could designate an army for PVP and or Defense of Alliance member/friend.
This way the re-enforcer would come in at that 19 minute period (as from anywhere on the map 19 minutes will give them enough time to get to any destination). It will pop up on the screen just like in the AI seiges. And making them controllable would be like making a co-op on an AI siege.
This would up the ante on actually attacking someones city and the attacker would have to be really on the ball attacking because same as AI enemy reinforcements are (if the attacker has already won the battle then the re-enforcements would become neutral).
Seek the Word or Embrace the Void!