I'm not familiar with Backpack's handling of rss feeds.
You could, of course, generate a key and do it that way (if I understand correctly). You could probably take that existing rss extension and just alter it to do that and release it yourself. You'd just need to program the key generation and add that value to the user's account.
Since there is as of yet no solution for getting usable e-mail notifications, I, too, been trying to get a RSS feed out of a forum that doesn't permit public browsing. The browser requires authentication, but returns the following error upon providing a user name and password:
A fatal, non-recoverable error has occurred Technical information (for support personel):
Error Message An error occurred while validating user credentials. Affected Elements Feed.ValidateCredentials();
The error occurred on or near: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(u.Password = md5('foobar') or u.Password = 'foobar' )' at lin