There really shouldn't even be a PrC that can do it. When you deal subdual damage with a weapon, you are swinging it in a way it just wasn't designed for (such as hitting someone with the flat of a sword blade). No matter how skilled the user, that is NEVER going to be as good as using the weapon in the intended fashion because the balance, etc. is going to be all wrong. It's like trying to cut things with a dull knife; you can use more force to compensate for the dullness of the blade but the same amount of force with a sharp knife would be more effective. You can't remove the penalty, only increase your ability so that you can do the job in spite of it.
If you need to subdue someone, either use a weapon designed for it (such as the sap) or use magic. Just because it is simple in terms of game mechanics to describe a certain effect (such as "removes the -4 penalty for attacking to subdue") doesn't mean that it makes sense to come up with a feat that has the effect.