I seriously doubt that the 40 man version of Naxx will or could ever be successfully multiboxed by one person. There are too many fights requiring precise movement, even for a level 70, and some fights with extreme DPS going on. There was a fight (Maexxna)where a tank basically had to fully flask and use stoneshields AND pop shield wall just to have a chance to survive the last phase. There are a couple fights where you had to MC tank the boss. There's a fight which requires 4 tanks rotating tanking even at level 70 in order to even live more than a minute or so. Etc etc etc. You could probably kill some trash, though.