You really dont need a mod for this.
There are forum permissions built into the admin control panel for each forum. Just set "delete" to admin-only for all places.
As for this restore forum, you can make it so mods can move topics (since you already would have disabled their delete abilities), and have them move topics to that special forum. Again, in the permissions, you can set it to only allow admin viewing, so you're good to go there again.
So, you see, phpBB can do this without any annoying code changes ^^