Yes. The same language that allows you to use Disarm, Trip, or Sunder as an attack of opportunity also would allow you to use Called Shot as an AoO. Unless there's some other obscure ruling I'd say you're good to go.
EDIT: would allow you to use *Greater* Called Shot as an AoO
EDIT: not to mention that the feat specifies you can use multiple Greater Called Shots in a *round*. Which necessitates the ability to use it outside of your turn, or else they would say multiple Greater Called Shots in your turn. Implications of word choice shouldn't be ignored - if dev's want to errata it they can, but as written you can use it in AoO's