Regarding the feed I'm pretty sure the answer is yes. I was able for example to import a pay walled feed into podverse for example. Then yes, maybe no one has asked subatack for PC 2.0 features so maybe this is possible. Honestly the most advanced thing they can do is just offer a rss snippet editor to customize at the channel/item level. Castapod does exactly this and it works pretty well.
Just a guess but I don’t think PI would let you add a private feed to the index. Sounds like a problem we would have solved very early on. You can add them into some apps but that’s so you can listen in the app instead of on a platform. I’ve done this with Patreon feeds for years. You could always ask substack to add features but honestly I wouldn’t hold my breath. If you really want PC 2.0 features that are easy to implement then go with something listed on https://podcastindex.org/apps?appTypes=hosting I recommend @podhome.fm and RSSBlue @dovydas b/c they are newer companies so they can build on the PC 2.0 spec faster since they are smaller and don’t have years of tech debit.
Thanks for the additional info. The request to substack would be kind of like an exit interview - if there are good notes around enough requests, and they end up with someone in place a year down the line who sees them and knows how to implement the RSS snippet editing, it could end up getting added to the platform and bring a lot of users in.