Just seen another topic which reminded me of a plugin I use to use.
What about a bump button, which updates posts, which also updates on the front of the forums, but doesn't actually post anything it just moved the whole topic to the top of the listing. Settings should include, limit time between bumps (Bump every 5 days, minutes etc), which groups can do it. The button should display next to the start new topic or reply button or replace it.