Although
I could have used this, I think it's a bad idea. I barely go back to read updates in the first post(s). Forum users read new posts, so if there's any news to add, add it as a new posts. For new participants, edit the first post and add the new changes.
For exmaple: In signature campaigns, the manager might tell us about new openings, OR in some big threads like those of MERIT history, Bitcointalk updates where big debates are going on, some good announcements to be made etc etc.
You can edit the OP with the latest additions, and quote that part as a new post to bring it to the attention of existing readers.