TLDR: A ~10 year old podcast with a well established audience (free and paid) wants a Roku channel, but only wants people to be able to login and view content that their subscription allows - no account management features, no signup, no subscription management in the channel, nothing fancy. Does the channel have to use RokuPay?
I have been approached about building a Roku channel for a podcast with a very large number of subscribers, some free, some paid, going back to the late 2000s. They (the podcast) do not want any account management features available in the Roku channel -- they want people to be able to sign in and view their content, and that's it. Anything other than signing in and searching/browsing/watching videos would take place on the podcast website, where their subscribers are comfortable and have experience.
If the channel is offering subscriptions on-device, they have to use Roku Pay. If they are not offering subscriptions on-device, they still may not direct customers to purchase subscriptions off-device (per requirement 2.3). Now that being said, implementing Roku Pay enables customers to purchase subscriptions with just a few keypresses. So that would seem like a missed oppotunity to maximize subscription revenue.
I would have to say that if you are only offering a login to the podcast on the channel and NO purchase/registration options, or directions off roku to register/subscribe on the channel then I don't see why you would not be able to. Just have your api return the content based on the credentials of the signed in user. Seems simple to me.