Have been thinking about the uses of AllowWeaponTarget in gadgets, but all the more powerful usages (overkill control) also require the ability to know which unit a particular shot is going for (which I still haven't figured how to detect)
Nevertheless, it would be logically simple (not UI simple though) to implement a global "ignore this unit" command. Allowweapontarget simply returns false for that player's units regarding the target, except when the player's units have a direct attack command.
Basically, my question is: Would the ability to make your units ignore a specific enemy unit except when directly ordered be useful?
Autoskirm/autoswarm/fight can be adjusted relatively easily to accommodate this as well.