[Plugin: bbpress] How to change 404 message for non-participants

I am running a child theme for Twenty-Twelve and using bbpress, designed for certain members only (it’s a scout site and the forum is for leaders, but not parents). Right now, non-registered visitors can view the forums page, but not the individual topics/posts. They get a custom “Restricted Access: You must be registered” blah blah page when they try to click on topics.

But once someone has registered, I am given the option of assigning their role. Leaders are Participants, and that works fine. Parents are not assigned any role (I’ve also tried using the Blocked role) — but instead of it taking them to the same page that non-registered visitors see, they are getting that “Well this is embarrassing” 404 page instead. (I hate that page.)

I’ve changed the text in the 404 area of my theme to read the same as the redirect page (Restricted Access you must be registered, etc.), but it’s still showing the Embarrassing page instead.

Is there a way to create a new role in bbpress that will redirect Parent users to the “Restricted Access” notice instead of the “Embarrassing” page? It’s frustrating that non-registered people can get the correct notice, but registered ones are getting that Embarrassing page.

Help? Thank you!