That’s an intruiging concept, I like it. It’s not an easy endeavour because you have to remove topics from the listings, counts and RSS (and not only stop users actually looking inside the topic). The Private Forums plugin has to do that too, so perhaps that’s a good starting point.
Of course, people can do what they want, but I don’t think this is an appropriate use of tags. Some plugin that added a text input field to the posting form and used the topic meta table to store the option would be better.