Sounds like its doable. Apparently you can embed Hackpad on your site using their code:
<script src=“https://hackpad.com/<string>.js”></script><noscript><div>View <a href=“https://hackpad.com/<string>.js”>To-Do</a></div></noscript>
So all you'd have to do is create a static page maybe and paste the above in a template
I know it is blasphemy to reply/reopen an old thread, but I think I have some sort of workaround for this. Say that per-category header warning message is 1) not going to be as native functionality, or 2) still waiting for plugin for that, I think a pinned thread atop all threads would suffice.