@tajhay one thing I should mention is that I wouldn't rely too much on your messages... simply because it only takes one person to submit something to the EasyList to block your specific message. While the session variable should always work, the actual message, if it uses any identifier, can be magically removed with most eye-dropping tools in the AdBlock extension.
You'll have to see what happens. I'm happy to try to help, but it's pretty difficult to get around it. You need to post your messaging using random identifiers at the very least. JavaScript works best, but even then, if it's loaded through XenForo it often gets a unique and specific identifier, especially in XF 2.3. This identifier makes it really easy for knowledgable AdBlock users to specifically target and remove the messaging (css class or ID). You'll need to keep an eye on it over time, and test periodically if you think the messaging is both important and imperative to be read by visitors.
You could probably use the sessionId() or something similarly random and unique per visitor. But even then, the actual message probably stays static, which again, can be targeted. Your mileage may vary. Not sure what else to say other than good luck
Note, I know you said you're doing something to posts, but just letting you know.
Let me know how it goes. I'm curious.