No, I don’t use the podcast feature on the Plex. But I do use Plex for listening to audiobooks. Just be aware that MP4/m4b cannot be in the same library as mp3s.
The other podcast thing is a solution totally outside of Plex. It is running on the same machine though and accessing the same files. dir2cast + webserver like Nginx or apache reads a directory of mp3 files and builds an RSS feed out of them. In some ways it works better than Plex because it’s simpler on the user side to listen offline as long as you sync the feed at home. I tend to do a separate feed for each series or author. It’s a bit fiddly to get setup and adding a new feed requires a bash one liner and editing some HTML after the files are sorted and named perfectly because podcast apps have some funny limitations when it comes to actually grouping, sorting, and displaying metadata.
Just in time for Google to kill RCS and move on to something else.