For example, much like <itunes:explicit> is not part of any RSS spec, a publisher could choose to include a tag like <castanet:monetize>yes</castanet:monetize>, which would tell the Cast-a-net app that the publisher of that podcast would like ads to be inserted. The publisher would then need to setup an account with Cast-a-net to share the ad revenue, verify ownership, etc.
There is a significant chicken and egg problem, of course. The player needs to have enough users for the publishers to consider setting up an account to be worthwhile. The ad experience also can't get so obnoxious that users move to other apps. This approach allows publishers to gain monetization and metrics without ceding ownership and control to the platform.
By the way, my player, Cast-a-net, doesn't yet offer this feature. I've been working on making the UI good enough to attract real users first, then hoping it can grow to be something worthy of specific attention from video podcast producers.
(edit: as was pointed out in a reply, Marco is very anti-ads, so I swapped the example tags. I had only meant to use Overcast as an example of a popular independent player that doesn't want to become a walled garden for just a subset of the total podcast universe.)