Not Used: Weapon focus: Falchion

I've brought on a fighter who specializes in Falchions, is there a weapon focus feat that he can use? If not, can one be enabled?

Unfortunately, there isn't a way to get the NWN engine to properly recognize feats like Weapon Focus and Weapon Specialization for non-standard weapons types (like CEP's falchion and Aenea's cestus). It's hardcoded into the game engine.

Understood.

What about modifying the falchion(s) and cestus baseline blueprints to avoid this penalty?

AB +1, Damage +2

This would also have the added benefit of encouraging their use.

For feat progression requirements PCs could specialize in an alternate weapon.

The biggest drawback is any bonus (either a pseudo feat scripting in a weapon focus effect, or applying the effect directly to the weapon) will be subject to the +20 AB cap, which is very reachable at epic levels.

Not used.

Because the weapon focus/specialization feats are hardcoded, a simple temporary attack or damage bonus added via script would not equal out.  Any attack bonus or damage bonus would become pointless if the weapon has a current attack bonus or enhancement bonus (they'd overlap, rather than stack like the feats).