I'm thinking more like moderators could enforce it, if they chose. it would be really valuable for support boards in particular. so, if the board was configured to do it, you would just see the option to post your message in an existing topic, like with radio buttons. so, say you post something about "merging topics", maybe your entire message (or just the topic title?) would be searched, and then you would see something like a draft screen, with radio buttons like so:
~ post this message to one of the following existing threads, instead of starting a new topic
~ blah blah blah
~ blo blo blo
~ this subject is not already being covered with an existing thread, post a new topic
...
i dunno, it might not be feasible, but it certainly is an interesting idea worth considering, no?