Why not just add it to the Reply page also. This way if the admin,moderator or user that wrote the main topic replies to the thread, they see a checkbox at the bottom that says mark this topic as solved. Then when they make a post, it will mark the topic as solved. It really does not have to be a BIG mod, but just change the title of the topic to [Resolved] {original name}. This mod should be able to be inserted in about 10 mins.