After a frustrating effort I can safely say I've tried everything I can think of to get my iPhone to subscribe to B sides. No dice, and I think that there's no solution in sight that gets B-sides directly onto our phones so long as it remains as a password protected feed because Apple says they won't list podcasts that are either sold nor password protected.(https://www.apple.com/itunes/podcasts/creatorfaq.html
So... here is my suggestion. I understand that you don't want to list the B-Sides feed under its real name because non-donors could so easily freeload. But how about listing the B-Sides feed in iTunes under a different, fake name that has nothing to do with the Drabblecast?
For example, list it as the "journal of cardboard box repair podcast" and then tell your subscribers to go subscribe to that feed.
That would seem to solve almost everyone's subscription problem without any additional risk. Any jerk could tell his friends about the unprotected feed, of course, but that same jerk could be sharing the common username and password for the feed right now.