Peter,
Not sure I completely understand why the module cannot use credentials to decide whether to show a module instance or not. It seems like that's the first test to do, then, make a choice about rendering a feed. I've always felt that using memory cache was preferable to reading from a database, but that's not always a workable model for a newsfeed, in my opinion. If a feed is fetched from the source URL, when setup, there is an opportunity to capture the relevant data and write it to a database table, which would include a TTL, either from the source, or calculated at the fetch time. Any subsequent attempt to access that information could be subject to authentication by the module permissions, and render if authenticated.
The background loading mechanism would be responsible for deciding whether to fetch an update using the TTL period, skip days, skip hours of the feed. If fetched again, the feed information is written to the database, and available to the next user. I know that some balk at using trips to a database to render a newsfeed, but I think it's better to fetch from a database, than to fetch from a newsfeed source which could possibly fail due to many reasons. Such failures could be trapped and the previous information is preserved, giving the user something to see, rather than give them nothing. Ajax could still be used to render the feed from a trip to the database. Yes, more trips to the database to read, but it beats the heck out of using Ajax to read from the source URL performance wise.
The problem with background loading is needing to time it. The scheduler only works when a user accesses the site, and fires off the checks. At least, that's what I've seen in my attempts to fetch news using it. So, that's another topic or facet to discuss.
Just a few thoughts, and I admit, I may be missing your point entirely, so we can continue to discuss the issues. I know many folks are interested in your thoughts, and appreciate the opportunity to shape the direction of the module. You know it's one of my favorite subjects.