Originally Posted by Jellby
I would not like a pop-up. It's annoying, and it only appears after one has already written the post (in that case, a likely reaction would be to post anyway, rather that write it again elsewhere).
I would prefer to replace the "Quick Reply" box (and "New Reply" button) with a warning, only by clicking away the warning (and hopefully reading it) should it be possible to write a reply.
I agree. This is the best idea so far if it can be implemented without excessive work.