As a workaround until you find a better solution, perhaps you could exploit the custom artwork feature.

If you have say "movie.mp4" then make two copies of a jpeg named "movie.jpg" and "movie-fanart.jpg" alongside the movie file. Plex will then show this.

You could generate the jpeg using ImageMagick or similar from a base image (per show say) and with text overlay.

I have some scripts that does this and it's better than nothing.