When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus.
Combat maneuvers are apparently attacks, and rolls made to attempt a combat maneuver are attack rolls. As such, they should be capable of producing a critical threat, which would then need to be confirmed, since the critical rules are a property of attack rolls.
This doesn’t appear to generally do anything, since combat maneuvers deal no damage, so there’s nothing to multiply, but it seems like it would “count” as a critical hit, so if you had something that says “whenever you score a critical hit…” (and there are several of those), it should trigger. Plus, one of the things you can do with a successful combat maneuver roll while grappling is to deal damage: if that roll was a critical, that damage might be eligible for multiplying.
But nowhere else in the rules seems to reference this. Natural 20s on the combat maneuver roll are explicitly mentioned, but no connection is drawn to the critical rules of attacks.
The questions I’m looking for answers to are as follows:
Is there anything I’ve missed, that disbars combat maneuvers from threatening criticals in the first place?
If your success on a combat maneuver roll results in damage (e.g. one of the options while grappling), is this damage eligible for multiplication if the combat maneuver roll was a confirmed critical hit?
How do you handle the critical threat range of the weapon they were using? Could a disarm with a rapier threaten a critical on a roll of 18? What about grappling while you had a kukri?
What is the target for the confirmation roll: their AC, or their CMD? (Unless someone shows differently, you would definitely continue to use CMB since the confirmation rules say you use the same bonus as on the original attack.)
Are there any official statements made about any of this? I have found a few forum discussions (Paizo’s forum, Giant in the Playground forum), but none seemed to result in developer involvement, or even in detailed consideration of the issue.
If no official statements are available, I would be interested in listening to descriptions of how you have handled this, or seen this handled, in one of your actual games that you have actually played. That is, I am not interested in speculation and I am not interested in how you think you’d rule it now that you think about it. But if this actually came up in a game, I’d be interested in how you answered these questions, particularly if combat-maneuver criticals were allowed to happen.
Combat maneuvers can't score threats
In the discussion of the Paizo Blog post Combat Maneuvers and Weapon Special Features Pathfinder designer Sean K. Reynolds in Oct. 2011 posted this question-and-answer:
To my knowledge, this hasn't been contradicted and remains Pathfinder's official stance.