I presume that to depart, all brake temps would have to be below the max specified. If 7 brake fans were operative then 7 wheels would have loads of capacity for any RTO as they are very cool with the eighth being at design limits, compared with the situation where all 8 wheels were already hot and at the design limits. I know what I would want, but then again I'm not an A330 driver!
But thats purely theoretical. I have no idea (obviously) when the MEL says!