Yeah the feeds needs a bit of love but I think it's doable.
I would start with:
Add de P20 namespace
<podcast:medium>audiobook</podcast:medium>
<guid>
And then try to improve:
<itunes:explicit>No -> true/false
Add publish date
Apple Category from Literature to Books
Podping / Auto submit to the Index
Some of the feeds are already on the PI so I think it a matter of talking to them, see if there is a git based system to help. And if the don't have it, pointing them to the right information, most if trivial stuff.
I was asking to be sure that this has not been tried before.
I only found a small story about them on Podnews podcasting news (npub1j34…e2z5) https://podnews.net/update/oxford-road-veritonic-combine#story2