The architecture of communication
evolutionistx.wordpress.com
evolutionistx.wordpress.com
But I'm not sure if anonymous boards are the solution. Without reputation, quality can quickly go down, and there is no incentive not to be needlessly nasty.
I think the best solution is for executives to actively solicit contrarian opinions via 1:1 meetings. An engineer might not want to speak out about why some project is doomed in a meeting of stakeholders. But if the exec comes to them privately and says, "give me your honest assessment, if this plan has holes in it, I want to know now," then they are likely to get an honest answer.