You can much more easily publish data via the blockchain w/o OP_RETURN, and furthermore, you can easily put that data in to the UTXO set which all nodes *must* have if they are to maintain consensus.
Mike Hearn suggested we adopt blacklists to solve this problem back when someone put the child porn sections of the hidden wiki into the UTXO set; no-one's come up with a better solution since. You can make publishing that data more expensive by a small linear factor - about 10x to 100x - but that's the best you can do.
The best solution to this problem is legal and political: the idea that you have to prevent every last trace of "illegal data" from getting into a public ledger is absurd.
Peter - your bandwidth is precious. Please don't waste time responding to moronic posts.
The OP should consider that his computer contains a sufficient amount of '1's and '0's which could all be simply rearranged somewhat to form 1000s of child porn images - ON HIS COMPUTER. But those '1's and '0's on his computer, just because they are presently out of order, nevertheless do constitute child porn. He is clearly a pig. That is right, your computer is loaded with child porn!!! The cops are going to find you and arrest you for this and you will go to jail for life.