when you logon to the site, (or come back if you choose the always log me in option)
as your browsing the site, you'll see the icons are RED on the forums.. this means a new post...
if you click that forum, the topic's icon is also red..
there is no room to really put this feature you describe here.. i know what you mean, but since this site is ONLY a forum, no main page there is no where to put the "new posts" section without making the forums main page clutetred..
and since this feature IS built in to phpbb, there is no reason to re-add it
use this legend as an example
No new Posts/Topics
Forum / Topic is locked