The act of attacking Marineford again was to get the news out. We also don't know what part the SH are actually understanding as the message. I agree though that if the purpose of the message was supposed to be the tattoo then Luffy did a poor job of showing it off. It did make the papers though which is the picture all the SH are most likely looking at. So if it's a visual message it'll be contained in that picture.
Let me cut to the point, because in this case I think the simplest answer is the most likely. From a story telling perspective, what is being discussed in this chapter and what is the point of it? I'm trying to make it clear that the tattoo is background, and for a small page in Shonen Jump barely even readable. Oda's telling a story, so if that played any part in the actual message don't you think he would make it more apparent or at least bring it up? No, all we get is Vivi questioning a fashion statement last chapter (tattoo). The World Government, Aokiji, Killer and Kid… they all talk about the bells and their meaning. That's the obvious news and message being conveyed to the readers and to the people in the story, so chances are high that was the message.