Has anything extensive been written about them in any of the supplements?
Beyond what little we know from GotF p. 99, nothing that I know of 
Thanks, I knew I had read it somewhere. And that's the book I had with me at work. 
Is there anything additional in the Bjornaer section of HoH:MC?
Anything in previous editions?
Yeah, but just bits and pieces, and from the Bjornaer view points (i.e. "they enslave the ancestor spirits," "they are spiritually corrupt," "they are generally doo-doo heads," etc.) . It references GotF for more information.
Edit:
It does sa the Ancestor spirits came to the Bjornaer founder asking for help.
maybe in Shamans
http://www.atlas-games.com/product_tables/AG0250.php
Though I havn't flicked through it for years. So unsure if it's what you're after