Benefit: Whenever you make a melee attack and hit your opponent, you can use a free action to grant that opponent a +2 bonus on attack and damage rolls against you until the end of your next turn or until your opponent attacks you, whichever happens first. If that opponent attacks you with this bonus, it provokes attacks of opportunity from your allies who have this feat.
Can this feat be activated follwing a successful attack of opportunity taken by me (against, say, an inbound opponent coming at me through threatened squares?
Free actions normally can't be used off your turn, but this is a free action following a melee attack. I'd say yes.
No. As SorrySleeping mentioned, free actions can only be take on your turn*. This means you can't use a free action when it's not your turn, such as when you take an AoO.
*There are some exceptions that are specified, such as if you have Snap Shot it allows you to draw an arrow (which is normally a free action).
Wouldn't this be a case of specific trumping general? The free action is part of the melee attack, which can happen off your turn.
Nope. It doesn't mention anything about being able to take it off your turn, so you can't.
There's no "specific" to override the general rule of "can't take free actions when it's not your turn".
Just because you got to make an AoO doesn't make it your turn.
As an exception to prove the point:
Free Actions: Can you take free actions during an attack of opportunity? For instance, can you use the Grab, Trip, Pull, or Push universal monster rules after hitting with an attack of opportunity, since they require free actions and free actions can’t be used off-turn? What about Rock Catching? That seems like it could only work off-turn.
While you can’t take most free actions off your turn, Grab, Trip, Pull, Push, and Rock Catching’s free actions can all be used off-turn. This will be reflected in future errata.