If I understand correctly, these people are using special software to encode data in transactions, namely the output addresses. You can only embed arbitrary data in blocks if you're able to mine a block (like Satoshi's headline in the genesis block and Dan Kaminsky's Ben Bernake ASCII art), otherwise you need to encode it in the transaction data.
What if A) your comment was preserved as a part of the block-chain that could not be removed without doing serious damage to bitcoin. And B) the cipher you are using is a publicly known cipher recognized as the standard way of encoding data into the block-chain.
That almost certainly cannot work. You'll have false negatives, people will sneak the words of synonyms in (just think of how many web forums has people saying "fcuk", or "f*ck"), and false positives, legitimate transactions being banned. Will anyone from scunthorpe be able to use it?
I would assume so.
Perhaps adding a word-checker to bitcoin clients that looks for recognizable words (or attempts to validate a URL found inside the most recent block).
If block contains words or URLs, disregard it. If not, accept it.
Now Namecoin on the other hand...