I have asked a number of instructors whether airbrakes can be used in the final turn. Some advise yea and some nay as the discussion above shows, and they all offer good logical reasons either way. The last instructor I asked said "I don't care where you use airbrakes" (I think the message was use them properly whenever needed).
In our motor glider/tug it is usual to set the brakes open to the first detent either on downwind or base and work them from that position.