I'm sure it must be possible. I'm a member of a dutch site which uses snitz forums. They have a RSS-feed, which works pretty simple...
- I get all messages from all forums
- I can read all guest-viewable posts without logging in, when I want to reply I can fill in my accountdetails (no separate login needed)
- If I try to read a post placed in a protected forum I get a page stating 'You have to be logged in to acces this page', with a link to login. When I click it I can supply my account details and get forwarded to the protected message. All of this within my RSS-reader.
Maybe you could have a look at this system. At the Snitz forum I found this
post which describes a system based on usergroups. Unfortunately I don't have acces to the RSS-mod itself.