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?)

It should be pointed out that doing this wouldn't be free. Fees are mandatory when transaction amounts are very small or the funds were spent recently.