FWIW, Giant Bomb (and I assume other sites) have premium podcasts that you can only access if you are a paid member[1], and you just subscribe using:

https://www.giantbomb.com/podcast-xml/premiumbombcast?api_ke... (you just copy it from the RSS button on their site)

(previously it was https://username:[email protected]/podcast-xml/prem...)

Seems to work across most apps. I don't see why it would be that hard to build a standard around something similar.

[1] primarily ad-free versions of their main podcasts, but they also have premium-exclusive podcasts