AJ and his agent will be instrumental in the creating a lot of terms of any contract - it won't be a case of clauses being inserted by Villa on the sly.
My guess is that the move to Burnley is made possible due to the lowish transfer fee - something I would guess the player is pushing for to get regular PL game time. In return, the club would like the option to buy the player back, with terms of this arrangement both pre agreed with the player and Burnley. Everyone would need to be happy with the this from the get go, otherwise there is no deal.
With regards to the lack of buy back clauses being optioned in the past, I would say that speaks volumes of just how few promising talents prove to be good enough to actually be recalled by a top team.
The Villa of old could never get away with this, as any promising youngster stood a good chance of game time. If the event arose that a talented academy player wanted regular football before there was a chance here, there's no way that if they proved successful they'd agree to come back to a consitently poor Villa team.
FWIW, I believe the only way AJ doesn't come back is if his progress doesn't match ours.