The problem with this idea is, IIRC the units the RTW engine can't disband the units, so basically you're giving the ai free units everytime you siege. This would not be a problem if it was a human player, as you can play with a house rule to always disband them after the siege is broken, but the computer cannot be made to do this. I mean, AS already manages to send full stack after full stack at you every turn without improved money scripts, imagine what this would do...