“Fake” a title for untitled posts

Any pointers for making a custom function which will test if an entry has a title and if not trim X words from the content and use that as a sort of placeholder? Mostly so that things such as the_post_navigation links get to have something other than “next post”/”previous post”, etc. I tried, for example, looking at how the Title Remover plugin interrupted the_title with a filter to see if I could suss out how to do what I’m looking to do, but so far no joy.