Technically, anyone who has contributed to the Linux kernel can drag them to court. Probably not something an average developer will jump into :) That said, Harald Welte has a dedicated site for this : http://gpl-violations.org/ and he is pretty active chasing violators. Personally, I believe if there is straightforward evidence that the sources they are providing is incomplete, spreading the word around is a good start!
As far as I could tell from that thread, there isn't, just a lot of ignorance. B&N has no legal obligation to tell them how to get a custom kernel to boot on the Nook.
