I guess I follow what you are saying in that WP is fetching the bbPress RSS feed via PHP. If your cookie method works in other cases, it should work with my plugin. But I don't think it's possible to send passwords via URL ("get") anymore in bbPress? This isn't a limitation of my plugin but of (newer?) bbPress.
By the way, kinda dangerous to send passwords that way as it will show in the logs.
In theory, you could show the feed via ajax (thereby using the user's password/browser) but that's probably more work that you want to do.
