Yes. You could also establish and publish a simple protocol for this, so that someone could run a program and extract all the hidden messages the way we can currently by looking for plain-text. Granted this would provide a means of detecting the messages (and therefore filtering them). This could be counteracted by first posting the data, then posting in the block-chain the fact that you posted the data (along with relevant meta-data). If you do this in a probabilistic way (IE, a single transaction has a 5% chance of being an advertisment of your data), then any would be filter would need to make a trade-off between false-positives, and ease at which a reader could operate.

Also, even when posted in plaintext, I wouldn't say that the messages are readable by people who are not looking for them. (How often do you look at the block chain?)