Depends on your use case, some documents are just not allowed to have random stuff in them. The best advertisement is just looking at properties and seeing Creator being Typst, if it’s a PDF. For printed ones, maybe a label on the back side of each/some pages can work. I also don’t see the point in central/standard way, it won’t work for other languages and might not fit in all cases (dimensions/formality).
Here are a few examples:
At best, in some documents you can add a (hidden) Easter egg somehow, in my experience.