Technically, a 51% attack would allow one to confirm non-standard transactions and pretty much do anything. Granted, this would immediately be apparent to any of the other 49% of nodes and cause a fork, but still...
Whats the point of that ? A miner can confirm non-standard transactions right now when he mines a block but no one would accept it. Point of 51% attack is to surreptitiously double spend and nothing else.
