This is actually possible with a little javascript help. (Note: this is inspired by https://mattwinckler.com/2015/11/plex-rename-a-tv-show-season/, which doesn’t quite work because jquery isn’t loaded.)
Copy the javascript code from this page and save it to a bookmarklet: https://gist.github.com/navels/13b904be70cb505b57789da9a18c99fb
(in Chrome, go to the Bookmark Manager, Organize, Add Page, name it whatever you want, and paste the above code into the location box).
Then in Plex when you have opened the season’s edit dialog, click the bookmarklet and it will add a Title input field, viola!
