I was thinking something like this:
It'd be implemented as a free feat for everyone. If you're using a polearm weapon while using it, the double damage is dealt on the first attack against the charging opponent, otherwise it's just normal damage.
First, the feat could be toggled on and off. If you move out of your spot while it's toggled, it toggles off. If you press the button to attack someone in melee, you are not moved out of your spot, but stay there in combat stance, much like the dwarven defender's defensive stance.
Then an attacker can move into your range. After your first attack, the feat is toggled off. You can't toggle it on again while an enemy is up close.
What determines the double damage with the halberd would be whether the one you attack was running as he entered your range. If he wasn't running, you'd just deal normal damage.