So after some mucking around, I found a great tool to do this.
You can tell it to download all the files in a playlist (your own or a channel), convert them to MP3 and then delete the videos. Just awesome. Next using @AndrewM command-line updater to Overcast, they appear in Overcast.
Some thoughts on automating this.
Have a cron job (scheduled task) on my mac mini (always on) to check every day (say 2 am) and
because youtube-dl has a playlist archive feature (it remembers which files have already been downloaded), download and add the files to a watch folder (once converted to MP3) that Hazel monitors and have it run an Applescript that calls the overcast uploader, using *.mp3 and they get uploaded. Have it send me an email off the output. All done!
Thoughts?