Plex has a lot of smart playlist options, but what I wanted was a playlist built out of the next unwatched epsiode from some selection of TV shows.

That's exactly what this script does. The variable playlistSearch takes search filters which you can get from going to "Advanced Filters" in the search.

The part you'd need to take out form the playlistSearch variable is going to be something like show.network%3D90469. In this specific case it filters for shows by CBC Television, though the specific network ID is not guaranteed to be the same on your system. Plex Tokens are also retrieved in the browser.

I run this script via a cron job on my Plex server. But it can be run anywhere that can access your Plex server.