Thanks for your time.
Getting into black magic between plugins here and I don't have too much knowledge on the right WordPressy way to go about this, so:
Let's say I have an arbitrary plugin that adds 3 custom fields to every Post, but it doesn't require them or offer a setting to do so, and I want them required without hacking that plugin. What is the correct WordPress approach to enforce that they are filled with something valid on Publish?