I’ve never done what your thinking but off the top of my head I’d suggest using local media assets → Local Media Assets - TV Shows | Plex Support. Example below …
Ozark - S01E01.mp4
Ozark - S01E01.jpg
The file Ozark - S01E01.jpg would be the artwork for the episode. You could probably script this easy enough to copy the file season background artwork for the episode. You might even be able to use symbolic if your on linux but I’ve never tried myself.
Once the local artwork is in place then just ensure you have Use local assets enabled on your library and force refresh the metadata to get it to pick it up.
BTW… depending if you have locked fields or not you might need to review this Reset all manual poster selections in a library at once or unlock other metadata fields as plex could lock the artwork if you edited it in the past blocking the local assets being picked up.
