My issue here isn't with the Mage Armor, so much as the Reinforced Clothes. The full Helm benefits stack with normal armor, so why not Reinforced Clothes? All they are is very, very light armor. The question is how to apply the bonus, so that it would stack like normal armor would; the Reinforced Clothes are still just Clothes, only they've got an AC modifier on them, so as far as NWN is concerned, they're magic, despite thier In-Game mundanity.
EDIT: I can already hear the comments about how making Reinforced Clothes act like regular armor in terms of Mechanics, would disrupt the game balance by making them too powerful, but... Really, now. Consider how much effort and skill goes into making just one set of even Iron Reinforced Clothes. And that's only the AC bonus of Padded Armor. All Reinforced Clothing is, is clothing, woven with a bit of metal so as to provide protection while still leaving the wearer totally unencumbered.