I should actually insert a 2b, which changes the "opponent knocked prone" to "opponent has effect applied" because I see no reason with Greater Trip wouldn't provide an AoO for someone using their Meteor Hammer to drag the opponent. Beyond that, I see no reason why someone can't use a Meteor Hammer to drag an already prone opponent, even if the Devs step in to say that you can't reapply the prone condition.
And frankly I'm closer to that position than anything.
A character with a meteor hammer and the Greater Trip feat absolutely can get an AoO for successfully tripping their target. This is further evidence of the order of operations: that the AoO occurs prior to the effect being applied.
This is supported within the rules (PRD:Combat:Combat Maneuvers:Determining Success). What is a successful combat maneuver? A successful die rol and an application of the effect. While normally occurring nearly simultaneously, these are two distinct "actions" that don't preclude the possibility of being immediately inturrupted under special circumstances.