What you might want to consider is that the special attacks all use CMB, not your BAB specifically (even though your BAB is part of the formula). This explains why there are no criticals for them; they aren't 'attack rolls' in the strictest sense (though that verbiage is in their descriptions). They also don't do damage, which is a requirement of criticals. This has been discussed elsewhere with spells IIRC.
All that said, you might go look for the critical hits deck for answers. Or you can just flavor the hits as you see fit:
Disarm: "With a well-placed blow you launch the guard's blade up into the air. He gapes at you stupidly for a moment as if he can't understand what has happened before reaching for his dagger. When he looks at you again, his jaw drops as his sword lands neatly in the palm of your outstretched hand." (You claim his weapon.)
Trip: "Your foe slams to the ground, his head making a hard thud as he hits. His eyes unfocus for a moment from the impact before he begins to rise." (Opponent's initiative roll decreases.)
And so on. Perhaps a bull-rush moves your foe an extra few feet, your overrun knocks your foe senseless like the trip, etc.
I know that it counts as an automatic success, but that seems like it isn't enough reward for getting the critical. Any thoughts?
I don't necessarily thinks there needs to be any additional reward. You can't really make anyone more disarmed than disarmed, or more tripped than tripped. Just like you can partially disarm someone one round and finish the job the next. HP isn't an all or nothing state like disarmed or tripped.
Generally, when someone rolls a crit on a skill check I describe it as happening with a flourish and leave it be. But there are so many situations where a skill check or special attack could roll a natural 20 that it is impossible to have general or specific rules for each.
If you want a general rule give the player a +2 circumstance bonus to their next d20 roll for their stunning success.
It has occurred to me that there is no game dynamic to explain what benefits (if any) there are to gaining a critical hit when you perform a special attack such as Disarm or Trip.
If you score a critical in normal combat you do additional damage, however what happens if you score a critical when attempting to Disarm or Trip an opponent? I know that it counts as an automatic success, but that seems like it isn't enough reward for getting the critical. Any thoughts?
One benefit would be that if you are using a bursting weapon when trying to sunder, the weapon would burst causing more damage to what you are trying to sunder, and possibly the wielder if you have Greater Sunder.