The ideal use case for hashtags is Luhmann’s famous Zettelkasten approach that made him one of the most prolific sociologists of the 20th century.
If you look at the way his Zettelkasten is structured, it could well be replicated in MarginNote with hashtags and links! A more powerful way to display / sort filter these relationships is almost all we need here:
I’m using this great tool to digitally recreate his approach. As it is open source you might be able to look at how to implement this fairly easily. He made a pretty nice presentation to help you understand his approach and the various ways of implementation.