We don't really develop MODS for phpBB3 here besides the ones you see already here. Icy Phoenix is based on phpBB2. If there is a Site Announcement MOD created for phpBB3, I'm sure it's posted somewhere on phpbb.com.
I will say that there is a
[MOD] ACP Site Announcement Centre built for Icy Phoenix available here.
This MOD would require you to be running Icy Phoenix instead of phpBB3, though.
If you are interested in using Icy Phoenix, follow this link to help you compare features of phpBB2, Icy Phoenix, and phpBB3:
http://www.icyphoenix.com/features.php