I noticed that there are new feats such as Bonett's Defense (kudos for the Princess Bride reference!) that are centered around having a one-handed weapon and nothing in the off hand. The NWN2 way of automatically making the one-handed weapon gripped two handed would break this from an IC standpoint. The "fencing" idea of an ac bonus is, I think, to reflect the smaller target area of a person standing side-ways on to their opponent with just the one weapon. Two-handing the weapon breaks this IC.
IF anything to allow two-handed grip of one-handed weapons were implemented, I would suggest making it a feat rather than automatic. A feat that is
activated like Power Attack, and when this feat is activated it disables the ac bonus from Bonettis' defense. Maybe, since it would then have drawbacks like having to buy the feat and disabling other feats, a non-magical attack bonus could be added to represent better control of a smaller weapon when you use two hands.
Double Grip
Activated
When this mode is activated, if you have a one-handed weapon equipped in your main hand and nothing in the off-hand, you gain +1 (non-magical) to your attack rolls and a physical damage bonus equal to 1/2 your Strength bonus (rounded down). If you have the feats "Bonetti's Defense", "Thibault's Geometry", or "Agrippa's Fundamental Guards", you loose the shield AC bonus while Double Grip is active.* If you already have a two-handed weapon equipped, or anything in your off-hand (including torches, flags, etcetera), Double Grip mode has no effect.
*When activated the feat not only checks to see if there is a one-handed weapon and nothing else, but also checks for those feats. For each of those three feats it encounters, it imposes a -1 shield ac penalty that goes away when Double Grip mode is de-activated. No need to re-write those other feats.
My only problem with this is that, since you can only have one combat mode active at one time, you cannot use your weapon two-handed while using Power Attack or Expertise, which makes the opposite of IC sense; two-handing a longsword should make both of those modes more effective, not impossible.
[EDIT]: Just had a thought: It could be a scripted implementation activated by an @ command so that it functions in power attack and expertise modes. Maybe still require purchasing a feat and still negate the Bonetti's Defense, etc, bonuses when active. Like @weaponstyle toggle whether the character is using their one-handed weapon with one hand or two.