Our forum was setup so that one thread = one HTML file. Any new replies to the thread were appended to the bottom of the file right before the footer. The added benefit of this is that it was very quick to read new posts as there was no processing taking place after the post is created.
Ex: If a new thread is being created, it will generate an ID, say 140585637400000000. Now this gets translated into a path : /140/585/637/400/000000.html and a file is created there.
The body of the new thread is added to the HTML file. Any subsequent replies are added to that same file right below the previous one.
So when a visitor requests example.com/topic/140585637400000000, the script takes the last part, turns into a file path, adds a template - which has the reply form with that ID - and sends it to the user. No additional processing needed. When a new reply is made, the script builds the path again and adds it to the bottom of the same HTML file. And so on...
I think your biggest hurdle is the initial planning. Try to carefully plan this out as much as you can, but obviously you won't know what future circumstances will bring. If you build it in such a way so that the storage mechanism doesn't need to change much or at all, you should be most of the way there.
Edit: Maybe I can convince the admin to publish some of the source. You're not the only one to be interested in something similar and I think there's a real demand for lo-fi community software.