So you want to have a forum to specifically make statements known to be injurious to the survivors or to express technical information that may be valuable to a lawsuit, but facially obscure it. Sounds like subpoenas would fly. Further, given users an illusion that they could say things in such a "hidden" forum that they might not say in open communication, just encourages bad behavior. Understand there's a difference between evidence and information that may be relevant to one party or the other. Further, in the case of a defamation or other action, such posts would indeed BE evidence.