I was frustrated by the RSS bug, too. Not much point in having private forums if all the info was readily available in the RSS feed, right?
Now, I'm not at all familiar with bbPress and I'm no PHP guru, but here's my assessment of what's going on. I would love to be corrected if I'm wrong!
To filter posts in the RSS feed, the plugin does this:
add_action( 'bb_rss.php', 'private_forums_filter_stuff_in_rss' );
The problem is that there isn't a bbPress hook called 'bb_rss.php'. Thus, the function never runs.
My solution (and I don't like it) is to add this line after every instance of the $posts array being populated in /rss.php:
do_action( 'bb_rss.php' );
That works, but any future bbPress updates will break it again.