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I'm sure it's already rendered to a static presentation-level (HTML/template language du jour) form at that point. That wouldn't work.

Just put some more javascript in there to deal with it. I'm sure it's not the hardest problem in the world.

Great, you've hidden the message with javascript, but now it's still in view-source and in search results for bots.

It's also still visible if someone is using noscript (actually, I have no idea if Facebook works with noscript, probably not).

The solution you are suggestion doesn't solve the problem and injects more corner cases.