It is simply impossible for someone to broadcast a transaction that does not follow the network rules to the network and expect anyone to accept it...
... 51% attack doesn't involve anything invalid or the breaking of rules.
I am looking at
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power. Are you saying that
Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
is not possible?